fix(ENG-RECORD-CONFLICT-SURFACES): retire MAX_README_CHARS, the fourth whole-file budget (#498)

[localai-bot]

README.md measured 29,993 characters against MAX_README_CHARS = 30000
(scripts/check-readme-structure.py:47). Seven free. The next README edit reds
CI, and two concurrent edits that each fit on their own merge into a file that
fails.

AGENTS.md Records forbids that shape in its own words, "Limit an entry, not a
shared file", and gives the reason: a shared-file budget forces each addition to
remove another entry, and merging two such edits cleanly is worse than
conflicting because it applies both removals.

This is the fourth budget of this shape, not a new argument

Budget	Retired by
per-class line limits	2026-08-10, recorded in AGENTS.md
MAX_CHARS in check-now-current.py + STATUS_RATCHET chars	87308dea under #364
PageRules.max_chars in check-public-doc-tables.py	#460 (PR #494)
MAX_README_CHARS	this PR, under #498

87308dea is the governing precedent and it is explicit about what survives: it
kept "every local per-cell and per-paragraph cap" while removing the whole-file
budget. This change mirrors it exactly.

What changed

MAX_README_CHARS and the branch that reads it are removed. The obligation
relocates rather than disappearing:

• MAX_CELL_CHARS = 220 unchanged, one table cell is one author's entry
• MAX_PARAGRAPH_CHARS = 900 unchanged, one prose paragraph is the largest unit
  one author writes alone
• required sections, ordering, em-dash, STATUS_LINK and CONTRIBUTOR_LINK
  unchanged, because they count defects rather than length and so cannot collide
  between concurrent pull requests

README.md is byte-identical at 29,993 characters. This PR does not add or
remove a single character of the landing page, so it cannot be confused with
buying headroom.

Rejected: a per-section budget

Issue #498 offers "a per-section or per-paragraph budget". A per-##-section cap
is rejected, and the spec records why: a section such as News is written by many
pull requests, so capping it recreates the same lock at smaller granularity. The
per-paragraph cap is the real entry cap and it already existed.

Evidence

Red-before, on the unmodified checker:

test_oversized_readme_passes_when_every_entry_is_small
  AssertionError: ['README is 69427 chars, over the 30000-char landing-page budget...'] != []
test_checker_declares_no_whole_file_budget
Ran 23 tests -- FAILED (failures=2)

The suite moves 21 to 23 tests, so the new cases demonstrably run. The retired
test_oversized_readme_fails is deleted because it pins the behavior being
removed; the two new cases plus the entry-cap guard replace its coverage.

Green after: Ran 23 tests ... OK, and check-readme-structure.py still passes
on the shipped, unchanged README.

Mutation, each applied to the committed tree and restored byte-for-byte
(sha256 asserted after every restore):

Mutation	Result
M1 reintroduce MAX_README_CHARS and its branch	2 tests RED
M2 MAX_PARAGRAPH_CHARS to 10,000,000	2 tests RED
M3 MAX_CELL_CHARS to 10,000,000	2 tests RED

M2 and M3 are the ones that matter for this change: they prove removing the file
budget did not quietly disable the entry budgets, which is the failure mode a
"the tests still pass" report would otherwise hide.

test_checker_declares_no_whole_file_budget asserts the constant is absent from
the module rather than checking its value, so the gate fails if it returns under
any threshold.

Full scripts/agent-preflight.sh: all gates green.

Also unblocks half of #1098

#1098 records that README.md cannot be corrected, and names two independent
blockers. Blocker 1 was this cap: the LTX-2.5 matrix row could only land by
deleting another architecture's row. That blocker is now gone, which also clears
the way for PR #1180's 37-to-40 architecture-count correction.

Blocker 2, check-doc-checkpoint.py refusing any README change that does not
also touch a landing source, is untouched and still owns #1098. No issue-index
row was appended here, because #1098 already has one and AGENTS.md forbids
editing an existing row.

Fixes #498.

FOLLOWING_AGENTS_PROTOCOL

Following-Agents-Protocol: true
AI-Assisted: true
Assisted-by: claude:claude-opus-5 [claude-code]