feat: add ignoreAssetChanges input to drop asset-hash-only changes

[laazyj]

Closes #162.

Problem

A Custom::CDKBucketDeployment's SourceObjectKeys changes on every content rebuild — the everyday case for a static site. cloudformation-diff classifies that custom-resource property change as MAY_REPLACE, so by default it is reported as an update and a destructive change. With the default failOnDestructiveChanges: true, that fails the job on every content-only PR, making the destructive gate unusable for any app that ships content via BucketDeployment (or otherwise has asset-hash-only churn).

This is also inconsistent today: evaluateDiff already exempts AWS::Lambda::Function whose only changes are Code/Metadata — but only for the counts (the comment body is still rendered from the raw TemplateDiff), and there is no equivalent for Custom::CDKBucketDeployment. See also #161 and #4.

What this adds

An opt-in ignoreAssetChanges input (default false). When true, any resource difference whose property updates are entirely asset hashes — Code, Metadata, SourceObjectKeys, CodeHash — is dropped from the TemplateDiff up front, so it is absent from the change counts, the rendered comment, and the destructive-change classification alike. Resources with any non-asset change are left fully intact.

The filter is applied once to templateDiff.resources (via DifferenceCollection.filter) before evaluateDiff/render, generalizing the existing hard-coded Lambda special-case into a documented option that also fixes the rendered body and the destructive gate.

Changes

• inputs.ts / .projenrc.ts (→ regenerated action.yml): new ignoreAssetChanges input
• action.ts: read it via getBooleanInput
• diff.ts: isAssetOnlyChange() predicate + dropAssetOnlyChanges(), gated on the flag in diffStack()
• stage-processor.ts: thread the option into StackDiff
• README.md: new "Ignore asset-only changes" section
• test/diff.test.ts: 3 tests — asset-only change present by default, dropped when enabled, and a mixed-change resource left intact

npx projen build passes (compile + eslint + 31 tests + bundle).

Live example

Dogfooded on a real repo. Same PR, toggling only this flag (identical content, identical deployed baseline):

ignoreAssetChanges: false → JasonDuffettNetSiteStack — 0 to add, 1 to update, 0 to destroy ❌, 1 Destructive Change (MAY_REPLACE on the BucketDeployment), SourceObjectKeys diff shown.

ignoreAssetChanges: true → JasonDuffettNetSiteStack — 0 to add, 0 to update, 0 to destroy ✅, no destructive changes, SourceObjectKeys filtered — while an unrelated Description change is retained (the filter is selective).

Before/after captured here: laazyj/jasonduffett.net#48

Notes / open questions

• Kept the existing default-on Lambda Code/Metadata count exemption untouched to avoid changing default behavior — happy to fold it into this mechanism in a follow-up if you'd prefer.
• Matching is by property name (the four asset properties), not resource type — simpler, and these names are conventional. Easy to tighten to resource-type-aware matching if you'd rather.
• Open to a different input name/shape — glad to adjust.