fix(rails): rail 14 is a monthly buy cap, not a fee-free allowance (#836)

[eaitbrahim]

Closes #836.

What was wrong

keel described rail 14's cap as the venue's fee-free allowance, and its research treated "inside the allowance" as a fee-free regime. keel's orders go through Coinbase Advanced Trade, which has no fee-waiver tier:

• On 2026-09-27 the account reported the Intro tier: 0.9% taker, 0.5% maker, has_promo_fee: false.
• Every live fill had paid a fee: 1.19%, 1.14%, 1.19% and 0.89%.
• Coinbase One's "zero trading fees up to $500/month" applies on the main Coinbase platform, not Advanced Trade.

What changed (wording only; enforcement untouched)

• keel/execution/guards.py has zero changed lines. The 29 rail 14 guard tests pass unchanged, which proves the cap, its fail-closed behaviour and every number it enforces are the same.
• No schema rename. The free_volume_usd field keeps its historical name. A rename would need a migration and would change no behaviour.
• Operator-facing text:
  • keel subscription set --help now says "Monthly BUY cap … not a fee waiver".
  • The throughput report says "route WITHIN each venue's attested buy cap … trades inside it still pay the venue's fee".
  • The significance verdict's zero-fee phrase reads "a zero fee (hypothetical: Advanced Trade orders are never fee-free, Rail 14: the "fee-free allowance" is not fee-free on Advanced Trade -- relabel it a monthly buy cap, and re-scope what rests on the premise #836)".
  • The simulator's tier section adds a paragraph saying its within-cap rows are hypothetical for keel's orders.
• Docs:
  • docs/rails/rail-14-subscription-allowance.md is retitled "the monthly buy cap". Its "What it bounds" section no longer claims a fee waiver or a "profitability boundary", and it carries a dated correction. The enforcement, veto and inspection sections are unchanged.
  • The runbook and the research toolkit are corrected in place.
• Records are appended to, never rewritten. Five records that described a fee-free regime each gain a dated Fee note under the title: the 2026-08-21 significance, 2026-08-22 trailing exits and 2026-09-05 restatement records, the quant-lab note cross-verification, and the 2026-09-27 accumulation-policy record. The accumulation note says its flat-taker run is the one matching live, and the verdict is unchanged.
• Code comments and docstrings now call the fee-free tier model hypothetical for keel's orders.

Tests

tests/test_rail14_is_a_buy_cap.py has 6 tests, all seen failing first:

• the help text,
• the throughput report,
• the significance label,
• the tier section,
• the rail 14 doc making no fee-waiver claim,
• every record that describes a fee-free regime carrying the Rail 14: the "fee-free allowance" is not fee-free on Advanced Trade -- relabel it a monthly buy cap, and re-scope what rests on the premise #836 note.

One existing test that pinned the old "fee-free" wording is rewritten to pin the corrected label.

Not in scope, deliberately

• The keel doctor fee-rate readout (Rail 14: the "fee-free allowance" is not fee-free on Advanced Trade -- relabel it a monthly buy cap, and re-scope what rests on the premise #836 item 4, marked optional there) is left for a follow-up.
• Switching the DCA to limit orders was decided against on 2026-09-27.

Verification

• Full suite: 6838 passed, 3 skipped.
• mypy, ruff check and ruff format --check are clean.

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