[Parser] Reduce expression/postfix grammar conflicts (phase 1)

[jbatte47]

Problem

The current grammar reports multiple parser conflicts, with a significant portion tied to expression/postfix parsing interactions (member access, call application, and precedence layering).

We need an incremental first pass focused on expression structure to reduce ambiguity without changing language behavior.

Scope

• Refactor expression grammar into explicit precedence tiers, e.g.:
  • primary
  • postfix (member access + call)
  • multiplicative/additive
  • flow pipeline (->)
• Remove ambiguous empty productions in expression-related paths where possible.
• Keep accepted syntax behavior compatible with current examples.

Deliverables

• Updated expression productions in sigil.y with clearer precedence.
• Reduced parser conflicts versus current baseline.
• Notes in PR describing which conflicts were targeted in this phase.

Acceptance Criteria

• Bison conflict count is lower than baseline for this phase.
• No regressions in current example parsing/execution.
• Member access and call chaining still behave as before.

Validation Checklist

• Capture baseline conflict report before change.
• Capture after-change conflict report and compare.
• Run all current examples/*.sigil.
• Add focused parser fixtures for nested call/member/flow combinations.

Non-goals

• Full elimination of all parser conflicts (handled in follow-up issue).
• New syntax features.
• Runtime semantics overhaul.