docs(manifesto): amend Principle 2 to acknowledge stdlib-depth tension

[danieljohnmorris]

Summary

• Adds an amendment paragraph to Principle 2 (Constrained) acknowledging that "small vocabulary" trades against stdlib depth
• Codifies a decision criterion for adding builtins: ≥3 independent persona transcripts as hand-rolled workarounds, each >40 tokens to express, with no existing composition that reduces below that threshold
• Links to persona-runs/ab-shared-issues.md as the living stdlib gaps register so the principle is grounded in measurement, not opinion

What changed

Single paragraph appended to the Principle 2 section of MANIFESTO.md. No other files modified. The amendment follows the same "acknowledged tension, not a contradiction" pattern used for hyphenated stdlib names (Principle 1) and residual English keywords (Principle 4).

Test plan

• Read MANIFESTO.md Principle 2 end-to-end — constraint principle still intact, amendment clearly scoped
• Decision criterion is concrete enough to triage a new stdlib-gap proposal against

Closes ILO-369

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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