openapi+irfacts: functions signal none or unbounded with an in-band -1 #1088

Problem

The styleguide bans in-band errors: no -1 for "not found". Five functions return -1 to mean "none" or "unbounded", and their callers test the sign:

typeGraph.link absorbs the irfacts pair: it skips an edge whose to is negative (typegraph.go:111). The callers at lines 64–73 hand it vertex and reference results unchecked, and line 76 tests v >= 0 itself.

git grep -nE 'return -1$' -- '*.go' ':!*_test.go'
git grep -nE '= -1$' -- '*.go' ':!*_test.go' ':!*/testdata/*'

The first command also finds annotation's sign, load's compareSites and jsonshape's compareConstants, where -1 is a sign or an ordering, not a sentinel. The second finds four named constants, each out of scope below, and two local loop markers that never leave their function (majorMinor's firstDot, firstWritten's at).

Direction

A draft.

• Each function returns (value, ok), as Go's own lookups do. remaining returns its count with a bool saying whether the budget bounds it.
• The typeGraph callers test ok before calling link, and link's sign guard goes.
• Each site is its own commit, in a pull request of its own: the file splits beside it (openapi: split merge.go and admits.go by responsibility #1095 for twoDigits) stay pure moves.

Out of scope

The four named constants holding -1 are values inside state, not results a caller must remember to test:

• ir.NoSource (ir/provenance.go) is the IR's declared "no source" value for Provenance.Source. It is part of the encoded document, settled in ir: pass and irverify disagree about Provenance.Source = -1 #127.
• load's foreign (load/validate.go:92) marks a finding's provenance until it is placed. It has ir.NoSource's value and lives in that same field.
• irfacts' unvisited (scc.go) is one index array's marker inside the strongly connected components search.
• irfacts' sharedProp (exposure.go) is one index array's marker inside the exposure index.

Done when

• git grep -nE 'return -1$' -- '*.go' ':!*_test.go' finds only the sign and ordering sites above.
• Goldens unchanged; go test ./... passes without -update.

Priority: P4, rule 1 (housekeeping: changes no lowering, validation, IR or emit-contract behaviour). The first command → 8 lines, 5 of them sentinels (morphic @ff8330ba).