fix(plan): pre-intern static string literals

[justinjoy]

Summary

• pre-intern static string literals from filter, projection, and aggregate expressions during plan generation
• prevent evaluation order from changing symbol IDs across otherwise identical easy sessions
• add plan-level and paired-session regression coverage for output-only projections

Root cause

WL_PLAN_EXPR_CONST_STR values were first interned by the evaluator. When a delta session evaluated a projection before its mirrored read session, its interner consumed extra IDs. A later host-provided symbol then received different IDs in the two sessions, causing the wyrelog engine pair to fail even though the underlying inserts succeeded.

Moving static literal interning into plan generation makes the IDs stable before either session evaluates the program. The serialized plan format and public API remain unchanged.

Validation

• meson test -C build wirelog:plan_gen wirelog:wirelog_easy wirelog:wirelog_advanced --print-errorlogs — 3/3 passed
• meson test -C build --suite abi --print-errorlogs — 32/32 passed
• standalone paired-session reproduction — read/delta late-symbol IDs remain equal
• wyrelog issue/614-explicit-permission-plane test-decide against the rebuilt library — passed

Closes #932