bug(bootstrap): BOOTSTRAP_HASH is blind to every policy action — JSON.stringify replacer misused as a key sort

## Problem

`computeBootstrapHash()` (`cdk/src/bootstrap/version.ts:32`) is intended to be an integrity digest over the bootstrap policy bundle. It currently digests **nothing but statement counts** — every action, resource, condition and effect is invisible to it.

```ts
const normalized = policies.map((p) => {
const json = p.toJSON();
return JSON.stringify(json, Object.keys(json).sort()); // <-- bug
});
```

The second argument to `JSON.stringify` is a **replacer / property allowlist**, not a sort comparator. `Object.keys(json).sort()` evaluates to `['Statement', 'Version']`, so serialization is restricted to top-level properties with those names — and because `Statement` is an *array*, its element objects are filtered to `{}`.

The payload actually hashed:

```
[
"{\"Statement\":[{},{},{},{},{}],\"Version\":\"2012-10-17\"}",
"{\"Statement\":[{},{},{},{},{},{},{},{},{},{},{}],\"Version\":\"2012-10-17\"}",
"{\"Statement\":[{},{},{},{},{},{},{},{},{},{}],\"Version\":\"2012-10-17\"}",
"{\"Statement\":[{}],\"Version\":\"2012-10-17\"}",
"{\"Statement\":[{}],\"Version\":\"2012-10-17\"}"
]
```

Each policy serializes to 41–71 characters. The comment above the function ("policies are serialized with sorted keys so that object property ordering does not affect the digest") describes an intent the code does not implement.

## Impact

**The hash cannot detect a policy change.** Demonstrated while adding grants on #165: I added `s3:GetBucketPolicy`, `s3:GetEncryptionConfiguration`, `sqs:AddPermission`, `sqs:RemovePermission` to two policies, regenerated artifacts, and `BOOTSTRAP_HASH` was **byte-identical**:

```
committed : b0501c8a57f20e4b5bf50d6fe8bbb934310adc4b64480228565d8392a19d1503
computed : b0501c8a57f20e4b5bf50d6fe8bbb934310adc4b64480228565d8392a19d1503
match : true ← after adding 4 IAM actions
```

`artifact-sync.test.ts`'s "committed BOOTSTRAP_HASH matches computed hash" therefore passes vacuously. Anything that preserves statement *count* — swapping an action, widening a resource ARN from a named prefix to `*`, flipping `Effect` from `Deny` to `Allow` — leaves the digest untouched. For a construct whose stated purpose is bounding IaCRole blast radius (RFC #120), that is the wrong failure direction: a silent widening is exactly what it should catch.

Only the count is protected, so adding *or* removing a whole statement does move the hash. That is why this has gone unnoticed.

## Fix

Serialize deterministically over the full document instead of misusing the replacer. Either sort keys recursively:

```ts
const stableStringify = (v: unknown): string =>
Array.isArray(v) ? `[${v.map(stableStringify).join(',')}]`
: v && typeof v === 'object'
? `{${Object.keys(v as object).sort().map(k =>
`${JSON.stringify(k)}:${stableStringify((v as Record)[k])}`).join(',')}}`
: JSON.stringify(v) ?? 'null';
```

…or use the replacer as intended (a `(key, value)` function that sorts object keys). Note `iam.PolicyDocument.toJSON()` output is already key-ordered by the CDK, so plain `JSON.stringify(json)` may be sufficient — worth confirming before adding machinery.

Bump `BOOTSTRAP_VERSION` in the same change: the digest necessarily changes for every existing bundle, so this is a one-time re-baseline, not drift.

## Acceptance criteria

- Adding, removing, or **altering** any action / resource / effect in any bootstrap policy changes `BOOTSTRAP_HASH`.
- A regression test proves it: mutate one action in-memory and assert the digest differs. (Today an equivalent test would fail.)
- `artifact-sync.test.ts` still passes with regenerated artifacts.
- The function's doc comment matches what the code does.

## Provenance

Introduced by #122 ("policies as typed TypeScript with version and hash"), the step that added the hash. Not caused by #165 — verified the same code on `origin/main`. Surfaced on #165 only because that PR is the first to add IAM actions since, and the unchanged hash looked wrong.

Related: #120 (RFC), #124 (resource-action-map), #125/#126 (the Aspect and live validator that will rely on this digest being meaningful).

Inizia da computeBootstrapHash() in cdk/src/bootstrap/version.ts e verifica come viene serializzato il JSON della policy e come viene definito BOOTSTRAP_VERSION. Esegui artifact-sync.test.ts, aggiungi la copertura di regressione descritta nell’issue e verifica che la modifica di un’azione cambi il digest, mentre gli artefatti rigenerati e la documentazione aggiornata rimangano coerenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.