refactor(validator): unify per-repo config override machinery

[plind-junior]

What

Collapses the three parallel per-repo config families in load_weights.py
(eligibility, scoring, time-decay) into one spec-driven mechanism. Each family
was added in a separate PR (#1293, #1300) by copy-pasting the same
resolve → coerce → parse → validate scaffolding, leaving the file at 666 lines
with three identical pick closures, two identical coerce functions, three
near-identical parse functions, six field-name tuples, and two hand-written
range validators.

Pure refactor — no behavioral change to scoring, eligibility, or config
loading. The public API (resolve_eligibility, resolve_scoring,
resolve_time_decay, all Repo*Config / Resolved* dataclasses) is unchanged,
so no downstream caller is touched.

Changes

• _FieldSpec: one declaration per overridable field — caster, default
  constant, range bounds (inclusive/exclusive), and an optional note (e.g.
  "used as a divisor"). Three tables drive everything: _ELIGIBILITY_SPECS,
  _TIME_DECAY_SPECS, _SCORING_SPECS.
• Generic helpers replace the duplicated machinery:
  • _resolve_overrides ← three pick-closure resolvers
  • _coerce_override ← _coerce_eligibility_value + _coerce_scoring_value
  • _parse_overrides ← three parse functions + six field-name tuples
  • _bound_desc / _validate_ranges ← two validators with ~15 inline checks
• resolve_*, _parse_*, and _validate_*_configs are now thin wrappers over
  the generic helpers.
• Net: −207 / +73 in load_weights.py. Adding a 4th override family is now
  a ~6-line spec-table entry instead of ~80 lines of boilerplate.

Tests

• 4 new cases in test_load_weights.py locking the centralized paths: the
  eligibility divisor case (exclusive-min + "used as a divisor" note), nested
  scoring.time_decay range rejection, and inclusive-vs-exclusive bracket
  rendering in error messages ([0, 1] vs (0, 1]).
• Full suite: 841 passing. ruff check, ruff format --check clean;
  pyright shows no new errors.