feat: Add fixed-point number data type proposals

This commit introduces Zarr extension proposals for binary fixed-point
numbers, based on the FixedPointNumbers.jl library.

-   Adds dedicated directories and READMEs for Qmfn and Nmfn fixed-point
    types across 8-bit, 16-bit, 32-bit, and 64-bit integer underlying
    types.
-   READMEs include detailed configuration, range, fill value
    representation, codec compatibility, Rust crate mapping (fixed and
    q-num), and maintainer information.
-   Renames 'fixed_point' directory to 'fixed-point' for consistency.
-   Adjusts the notation explanation to consistently use 'n' for
    fractional bits and clarifies Rust crate mappings.
-   Adds a note about potential 128-bit extensions.

Author: mkitti

data-types/fixed-point/N0f16/README.md

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+# N0f16
+
+An unsigned 16-bit "normed" fixed-point number with 16 fractional bits.
+
+**Status:** proposal
+
+## Data type name
+
+`N0f16`
+
+## Configuration
+
+This data type represents an unsigned "normed" fixed-point number based on an underlying 16-bit integer.
+- **base_type**: `uint16`
+- **f**: 16 (number of fractional bits)
+- **scaling**: by `2^f - 1`
+- **range**: `0.0` to `1.0`
+
+The stored `UInt16` value `i` is interpreted as `i / (2^16 - 1)`. This is equivalent to `Normed{UInt16, 16}` in `FixedPointNumbers.jl`.
+
+## Fill value representation
+
+The `fill_value` for this data type should be represented as a floating-point number in the JSON metadata.
+
+## Codec compatibility
+
+This data type is stored as an `uint16`. It is expected to be compatible with any codec that can handle the `uint16` data type.
+
+## See also
+
+- [`fixed_point/` directory](../) for other fixed-point types.
+- [FixedPointNumbers.jl](https://github.com/JuliaMath/FixedPointNumbers.jl)
+
+## Current maintainers
+
+- Mark Kittisopikul (@mkitti), Howard Hughes Medical Institute

data-types/fixed-point/N0f16/schema.json

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+{
+    "name": "N0f16"
+}

data-types/fixed-point/N0f32/README.md

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+# N0f32
+
+An unsigned 32-bit "normed" fixed-point number with 32 fractional bits.
+
+**Status:** proposal
+
+## Data type name
+
+`N0f32`
+
+## Configuration
+
+This data type represents an unsigned "normed" fixed-point number based on an underlying 32-bit integer.
+- **base_type**: `uint32`
+- **f**: 32 (number of fractional bits)
+- **scaling**: by `2^f - 1`
+- **range**: `0.0` to `1.0`
+
+The stored `UInt32` value `i` is interpreted as `i / (2^32 - 1)`. This is equivalent to `Normed{UInt32, 32}` in `FixedPointNumbers.jl`.
+
+## Fill value representation
+
+The `fill_value` for this data type should be represented as a floating-point number in the JSON metadata.
+
+## Codec compatibility
+
+This data type is stored as an `uint32`. It is expected to be compatible with any codec that can handle the `uint32` data type.
+
+## See also
+
+- [`fixed_point/` directory](../) for other fixed-point types.
+- [FixedPointNumbers.jl](https://github.com/JuliaMath/FixedPointNumbers.jl)
+
+## Current maintainers
+
+- Mark Kittisopikul (@mkitti), Howard Hughes Medical Institute

data-types/fixed-point/N0f32/schema.json

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+{
+    "name": "N0f32"
+}

data-types/fixed-point/N0f64/README.md

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+# N0f64
+
+An unsigned 64-bit "normed" fixed-point number with 64 fractional bits.
+
+**Status:** proposal
+
+## Data type name
+
+`N0f64`
+
+## Configuration
+
+This data type represents an unsigned "normed" fixed-point number based on an underlying 64-bit integer.
+- **base_type**: `uint64`
+- **f**: 64 (number of fractional bits)
+- **scaling**: by `2^f - 1`
+- **range**: `0.0` to `1.0`
+
+The stored `UInt64` value `i` is interpreted as `i / (2^64 - 1)`. This is equivalent to `Normed{UInt64, 64}` in `FixedPointNumbers.jl`.
+
+## Fill value representation
+
+The `fill_value` for this data type should be represented as a floating-point number in the JSON metadata.
+
+## Codec compatibility
+
+This data type is stored as an `uint64`. It is expected to be compatible with any codec that can handle the `uint64` data type.
+
+## See also
+
+- [`fixed_point/` directory](../) for other fixed-point types.
+- [FixedPointNumbers.jl](https://github.com/JuliaMath/FixedPointNumbers.jl)
+
+## Current maintainers
+
+- Mark Kittisopikul (@mkitti), Howard Hughes Medical Institute

data-types/fixed-point/N0f64/schema.json

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+{
+    "name": "N0f64"
+}

data-types/fixed-point/N0f8/README.md

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+# N0f8
+
+An unsigned 8-bit "normed" fixed-point number with 8 fractional bits.
+
+**Status:** proposal
+
+## Data type name
+
+`N0f8`
+
+## Configuration
+
+This data type represents an unsigned "normed" fixed-point number based on an underlying 8-bit integer.
+- **base_type**: `uint8`
+- **f**: 8 (number of fractional bits)
+- **scaling**: by `2^f - 1`
+- **range**: `0.0` to `1.0`
+
+The stored `UInt8` value `i` is interpreted as `i / (2^8 - 1)`. This is equivalent to `Normed{UInt8, 8}` in `FixedPointNumbers.jl`. This maps the integer range `0..255` to the floating point range `0.0..1.0`.
+
+## Fill value representation
+
+The `fill_value` for this data type should be represented as a floating-point number in the JSON metadata. For example: `"fill_value": 0.5`.
+
+## Codec compatibility
+
+This data type is stored as a `uint8`. It is expected to be compatible with any codec that can handle the `uint8` data type.
+
+## See also
+
+- [`fixed_point/` directory](../) for other fixed-point types.
+- [FixedPointNumbers.jl](https://github.com/JuliaMath/FixedPointNumbers.jl)
+
+## Current maintainers
+
+- Mark Kittisopikul (@mkitti), Howard Hughes Medical Institute

data-types/fixed-point/N0f8/schema.json

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+{
+    "name": "N0f8"
+}

data-types/fixed-point/N10f22/README.md

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+# N10f22
+
+An unsigned 32-bit "normed" fixed-point number with 22 fractional bits.
+
+**Status:** proposal
+
+## Data type name
+
+`N10f22`
+
+## Configuration
+
+This data type represents an unsigned "normed" fixed-point number based on an underlying 32-bit integer.
+- **base_type**: `uint32`
+- **f**: 22 (number of fractional bits)
+- **scaling**: by `2^f - 1`
+- **range**: `0.0` to `1024.0002439`
+
+The stored `UInt32` value `i` is interpreted as `i / (2^22 - 1)`. This is equivalent to `Normed{UInt32, 22}` in `FixedPointNumbers.jl`.
+
+## Fill value representation
+
+The `fill_value` for this data type should be represented as a floating-point number in the JSON metadata.
+
+## Codec compatibility
+
+This data type is stored as an `uint32`. It is expected to be compatible with any codec that can handle the `uint32` data type.
+
+## See also
+
+- [`fixed_point/` directory](../) for other fixed-point types.
+- [FixedPointNumbers.jl](https://github.com/JuliaMath/FixedPointNumbers.jl)
+
+## Current maintainers
+
+- Mark Kittisopikul (@mkitti), Howard Hughes Medical Institute

data-types/fixed-point/N10f22/schema.json

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+{
+    "name": "N10f22"
+}