fix(analyzing-weights): bind weight changes to the correct function via -W

[manuelmauro]

What & why

analyzing-weights's analyze-weight-diff.py was mis-attributing weight changes to the wrong functions and silently dropping flags.

The parser identifies each change's function from (1) the git @@ hunk header and (2) fn NAME(...) lines in the diff context. For Substrate weight files both fail:

• Hunk headers show the enclosing impl ... WeightInfo block, never a fn.
• The fn NAME(...) signature sits ~5 lines above the first changed line (// Minimum execution time / Weight::from_parts), outside the default 3-line context — so it usually isn't in the diff at all.

With neither available, current_fn stops advancing and changes from several consecutive functions collapse into one bucket; parse_weight_block then keeps only the last value per variable.

Observed impact on a full runtime weight regen (moonbeam #3767):

• A per-y 3.9K → 18.8K (+386.9%) change was reported against pay_one_collator_reward_best, but it actually belongs to delegate_with_auto_compound (verified in source: 3_861 → 18_800).
• Reported 50 changed functions / 7 Section-3 flags; the real numbers are ~1,040 functions / 22 flags.

Fix

• Require git diff -W (--function-context) so the fn signature is always present in the diff. Documented in both the script docstring and SKILL.md (step 2), with an explanation of why it's needed.
• Warn loudly when a bucket merges multiple functions (detected via >1 Minimum execution time line per bucket — the tell-tale of a diff produced without -W) instead of emitting wrong numbers.

Verification

• Without -W: prints the new warning (33 merged buckets on #3767).
• With -W: no warning, 1,037 functions, correct per-function attribution (spot-checked against source).