feat: inherited when ... from agent attribution (Invariant prerequisite, v2 §43)

[rmichaelthomas]

Summary

Implements Invariant Checkpoint v2 §43 (IRQ-3) — the one upstream prerequisite for liminate-invariant. Extends statement-final agent attribution to inherited when headers.

Locked grammar:

inherited when <condition> [unless <guard>] from <agent-name> [:]

• Legal only with the inherited prefix. A trailing from on a plain when header remains a parse error.
• Agent-name rules are identical to existing statement attribution: single UNKNOWN token, hyphenated names, quoted strings rejected, reserved words rejected.
• inherited_from flows into HANDLER_FIRE trigger metadata so downstream consumers (Invariant) can attribute Narratia-written handlers (inherited when ... from narratia) across sessions.
• No new vocabulary word — reuses the MS-Q4 from reuse for one more grammatical position. The 60-word count is unchanged.

Phases completed

• Phase 1 (parser) — parse_when_block consumes from <agent> attribution after the optional unless guard and before the optional colon, gated on is_inherited. A non-inherited when with a trailing from now raises a targeted parse error directing the user to the inherited prefix. Reuses _try_consume_inherited_from unchanged.
• Phase 2 (renderer) — _render_when appends from <agent> to the header line (never after the multi-line action block). render()'s generic statement-final suffix is guarded against WhenNode so it doesn't double-append.
• Phase 3 (tests) — 12 new tests in tests/test_inherited.py (45 → 57): parse success (bare / with guard / with colon / with of-condition), all four error paths, header-line rendering, round-trip fidelity (asserting compare=False fields explicitly), and an end-to-end HANDLER_FIRE trigger-metadata integration test through the actual listen() generator.

Files modified

• src/liminate/parser.py
• src/liminate/renderer.py
• tests/test_inherited.py

Invariants satisfied

1. Round-trip fidelity holds for attributed inherited when nodes.
2. Plain when is untouched — trailing from still errors; without it, behavior is unchanged.
3. inherited_from is inert provenance metadata (compare=False), never read by execution.
4. Single agent-name ruleset — no forked validation logic.
5. No vocabulary change — vocabulary.py untouched, word count remains 60.

listener.py required no change — _wrap_with_trigger already read handler.when_node.inherited_from (built dark, ahead of this grammar landing); this PR is what gives it a non-None value to carry for the first time.

Test plan

• Full suite: python3 -m pytest — 1511 passed, 0 failed, 0 regressions (was 1499 before this branch)
• All Phase 1/2/3 acceptance criteria from the build prompt verified individually
• Rendered canonical form confirmed to place attribution on the header line:

  inherited when level is above 50 unless level is above 90 from compliance-agent
    show level
  

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