fix(journal): raise event ceiling with bounded runtime reads

[solomonneas]

Summary

• raise the lifecycle journal ceiling from 512 to 2048 events while keeping the 8 MiB byte bound
• warn at 75% event usage and allow runs stopped at the old ceiling to recover and append from sequence 513
• route runtime mutation and control paths through the shared bounded reader
• store the measurement artifact, correct its headroom arithmetic, and fail closed on unreadable or incomplete inputs

Why

Measurements showed that event count reaches the old ceiling well before journal bytes become binding. Raising the shared event bound preserves the current single-file journal design while the 75% warning gives operators time to react before append refuses a transition.

Verification

• journal, measurement, lifecycle, and shadow suites: 193 passed on the code-equivalent tree before the documentation-only docs(contributing): document enforced merge grading gate #667 merge, receipt 20260802-013725-work-verify-512672
• ./scripts/verify: passed on that tree, receipt 20260802-013803-work-verify-1820ab
• independent review reported no blocking findings and identified 2 test-hardening changes that are included in the final branch

Closes #653
Closes #635

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