fix(check): read the ~ operator sites the parser records instead of walking the tree

[devin-ai-integration]

What and why

UndefinedOperatorPass found every ~x by running ast.Inspect, a reflection-based walk over every node of every document, on each analysis. A CPU profile of BenchmarkLoadModel/elements=1000 put that walk at 7.6% of total load time, about 40% of the validate slowdown against 0.8.1, for a warning that almost never fires.

The parser is the one place that builds an OpBitNot expression (parseUnary), so it now records those sites and the pass reads the list:

type RootNamespace struct {
    Members            []Node
    UndefinedOperators []*OperatorExpr // `~` expressions in source order
}

• Parser.undefinedOps is appended in parseUnary; parseCheckpoint carries its length and restore truncates it, so an abandoned try-parse leaves nothing behind; ParseFile hands the list to the root.
• astcodec encodes/decodes the field for RootNamespace (typed pointer slice, same pattern as regions/ends), so the on-disk index cache keeps it. The embedded stdlib.snapshot is regenerated for the format change.
• UndefinedOperatorPass.Run iterates root.UndefinedOperators; level, code and message are unchanged.

ast.Inspect stays: the export tests, hygiene tests and the doc-counts census still use it.

BenchmarkLoadModel/elements=1000 (-benchtime 20x, -count 3): develop 108.2/111.8/120.8 ms → 103.2/103.3/106.6 ms, about 7% faster.

How it was verified

• New TestUndefinedOperators* in internal/syntax/parser: source order, nested ~~x, positions inside calc bodies and invocation arguments.
• New tests/parser fixture undefined-operator.sysml (golden added) and TestUndefinedOperatorsCoversFixtures, which parses every parse fixture and checks the recorded list against an independent reflective count of OpBitNot nodes.
• Existing undefined_operator_test.go passes unchanged; astcodec round-trip/sharing tests pass.
• gofmt -l . empty, go build ./..., go vet ./..., go test ./internal/syntax/... ./internal/check/... ./tests/parser ./internal/workspace/..., TestStdlibConformance, python3 scripts/changelog.py check.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring