Add 100-word report for GH#62787; fix line endings via pre-commit

Author: Mayank

REPORT_62787_100W.md

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+# GH#62787: CoW replace with dict containing np.nan
+
+Issue: With Copy-on-Write enabled, DataFrame.replace({np.nan: ...}) via dict raised ValueError about a weakref not in list. Root cause was in Block.replace_list CoW cleanup: dead weakrefs (b() is None) were included in the referenced_blocks lookup, leading to invalid pops during multi-step replace.
+
+Fix: In core/internals/blocks.py, build the self_blk_ids map only from live weakrefs:
+
+- Skip entries where b() is None.
+- Preserve existing CoW semantics; no API change.
+
+Tests: Added tests covering dict replacements with np.nan/NA/NaT, Series behavior, inplace, mixed dtypes, nested dicts, chaining, and CoW vs non-CoW parity.
+
+Result: Pre-commit clean; CI expected to pass.