docs(parser): give Gate G's allowlist its owner, its rationale, and its exit condition

[matthewevans]

Ruling rider 1 on task #122. Gate G already enforced the exact-set property; what
it did not do was say WHY each survivor is on the list, WHO closes it, or WHEN the
gate retires. A ratchet whose entries carry no rationale decays into a blessing —
the next reader sees a passing gate and a magic number, not a work-list.

Each of the 16 permissive router calls is now enumerated with its file:line, its
defect class, and the task that owns it (#123):

• priority 0 / 1b (:3720,:3725,:3789) commit on Some(vec![]), which
  extract_granted_keyword_list returns on MTGJSON metadata ALONE — the
  parameterized Oracle line is never parsed.
• the flashback split (:4673,:4682) is NON-ATOMIC: it advances when only one
  half parsed.
• priority 8f (:5154) is the worst: Class-A, i += 1; continue; OUTSIDE both
  if let Some blocks, so an unparseable line is consumed with no keyword and
  no Unimplemented — and it dispatches on a bare alt((tag("kicker"), …)) with
  no word-boundary guard.
• suspend/specialize/buyback/escalate/commander-ninjutsu/p13-residual/d20
  (:5092,:5102,:5170,:5186,:5521,:5554,:5646) are advance-on-partial.
• the two classifiers (:2161,:2167,:2205) gate ROUTING decisions on a permissive
  parse. Step 5 item 11: a classifier may probe with the STRICT parser; it may
  not call a helper that discards the remainder.

DEFINITION OF DONE is now stated in the gate itself: entries are DELETED as each
router entry converts, and when every EXPECTED_* reaches 0 this becomes the plain
"no permissive symbol in a router context" gate Plan 02 step 7 actually asks for,
taking SKILL.md §3a's migration-status paragraph with it. The down-drift failure
message now names #123 and requires the floor to be lowered in the SAME commit, so
a partial migration cannot land while the gate still advertises the old count.

Exact-set property re-witnessed in BOTH directions after the edit:
add a permissive call to a scratch oracle.rs (13 -> 14) => EXIT 1
migrate one away without lowering the floor (13 -> 12) => EXIT 1, names #123
restored => EXIT 0
oracle.rs byte-identical afterwards. No production code touched.

[gemini-code-assist]

Code Review

This pull request updates the scripts/check-parser-combinators.sh script to expand documentation on the parser-combinator ratchet mechanism, adding a detailed allowlist of permissive calls and clarifying instructions for task #123 when migrating entries to the strict parser. There are no review comments, and I have no feedback to provide.

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