Strip trailing separators from matchedPrefixLength; simplify directionSensitive

[curtisman]

Summary

Strip trailing separator characters from matchedPrefixLength (P) so it always
lands before the flex-space, aligning wildcard→keyword transitions with
keyword→keyword transitions. Simplify directionSensitive to P > 0.

Changes

Core: grammarCompletion.ts

• stripTrailingSeparators helper — scans backward from a position,
  stripping separator characters down to a lower bound.
• Phase B1 (backward) — partial keyword positions are stripped before
  advancing maxPrefixLength, so the backward anchor sits at the last
  non-separator boundary.
• Phase B2 (forward) — anchor is stripped before the displace/merge
  decision. The old isSeparatorOnlyGap check in Phase B2 is replaced:
  stripping collapses separator-only gaps naturally (anchor = maxPrefixLength
  → merge), while non-separator gaps remain above maxPrefixLength (→ displace).
• mergeSepMode helper — combines the old computeNeedsSep +
  mergeSeparatorMode two-step into a single function, reducing boilerplate
  at every call site.
• directionSensitive simplified — from
  openWildcard || P != minPrefixLength to P > 0.
  minPrefixLength is a caller-supplied floor, not a result property.
• Named type WildcardStringRangeCandidate replaces inline
  Extract<RangeCandidate, {kind: "wildcardString"}>.

Tests

• Updated matchedPrefixLength expectations (−1 for trailing separator no
  longer counted) and corresponding separatorMode changes
  ("optional" → "spacePunctuation" where separator is now required).
• One directionSensitive: false → true for the minPrefixLength floor case.
• Cross-direction invariant checks now pass undefined for minPrefixLength.
• New E2E scenarios in grammarE2E.spec.ts: wildcard→keyword trie
  narrowing, backspace round-trip, double-space, wildcard→keyword→entity
  pipeline. Extracted primeWildcard() helper for DRY setup.

Documentation

• actionGrammar.md — updated Phase B1/B2 descriptions, directionSensitive
  decision tree, and design-choice rationale.
• completion.md — updated separatorMode, directionSensitive, and
  decision tree sections.