feat: carry the document's declared constraints into the derivation

[ShocOne]

Why

A document states more about a property than the derivation could see.

format was parsed but reached only the audit's value synthesiser
(audit/plan/synth.go, audit/run/strategize.go) — the IR flattener's flat
struct did not carry it, so format: password was structurally unable to
influence a generated schema. writeOnly, deprecated, uniqueItems,
minLength, maxLength, minItems and maxItems were not parsed at all:
specmodel's decoder is lookup-driven, so they were silently dropped.

This is the foundation three separate pieces of work need — Sensitive
(mapping.md row 4), the plan-time validators (row 9), and SetAttribute
(row 11) — and it is also what the date-time fixture defect uncovered in #89
turns out to require.

What changed

• specmodel.Schema gains WriteOnly, Deprecated, UniqueItems,
  MinLength, MaxLength, MinItems, MaxItems. The four flags decode
  through one table, sharing readOnly's error message shape; the bounds share
  the existing numeric-bound table's.
• flat gains all of them plus format, pattern and the numeric bounds.
  Bounds fold first-wins through foldBound, matching how the description
  and the enum already fold: a later allOf branch restating a bound states
  nothing new.
• ir.Attribute gains Format, WriteOnly, Deprecated, UniqueItems,
  Pattern, Minimum, Maximum, MinLength, MaxLength, MinItems,
  MaxItems, all omitempty.

Constraints are taken from the write side: a constraint is a rule about what
may be sent, and a response schema restating it says nothing extra. WriteOnly
comes from the create side only — a response schema cannot declare it.

Nothing reads them yet. That is deliberate: this PR is parse-and-carry, so
it can be reviewed against a byte-identical output.

Measured effect

Generated output is byte-identical on all three pilots. Refusals are exactly
main's:

	before	after
github	937	937
jamfpro	319	319
thousandeyes	344	344

tfpfgen provider verify — no drift on all three.

What did change is --print-ir, which now carries the facts:

field	github	jamfpro	thousandeyes	total
format	7,355	810	757	8,922
write_only	—	36	127	163
deprecated	70	38	71	179
unique_items	2	2	33	37
min_length / max_length	16 / 47	124 / 54	—	140 / 101
minimum / maximum	2 / 29	57 / 36	112 / 196	171 / 261
min_items / max_items	11 / 19	—	— / 1	11 / 20
pattern	11	14	—	25

Formats that now reach the derivation include password ×43 (jamfpro),
date-time ×1,543, uuid ×39, uri ×5,640.

Those password and write_only counts are the security-visible defect
handoff.md:107 names: they generate as plain, visible attributes today. The
next PR acts on them.

Gates

• gofmt -l internal cmd — empty
• scripts/repo_hygiene_gate.sh — pass
• golangci-lint run — 0 issues
• go build ./... && go vet ./... — pass
• go test ./... — pass, coverage 91.3% total, internal/specmodel 95.0%
• tfpfgen provider generate (postcheck) + provider verify on all three — no drift

New tests: internal/specmodel/constraints_test.go,
internal/intermediate_representation/constraints_test.go, and three refusal
cases for a non-boolean writeOnly / uniqueItems and a non-numeric maxLength.

🤖 Generated with Claude Code