fix(amplify-velocity-template): consistent handling of reserved property names in reference resolution

[sarayev]

Follow-on to #14964, which made the VTL reference resolver skip a set of reserved property names but left three code paths that still resolved or wrote those names. This makes the handling consistent across all of them.

What changed

1. getter() case-normalized lookup (src/compile/references.js) — getter() only checked its first base[property] lookup. When that returns undefined, it re-cases the property name (for example getConstructor() -> Constructor -> lowercased constructor) and does a second lookup with no check, so a reserved name still resolved through the re-cased path. Added the same reserved-name check before both lookups.

2. Write branches in getPropMethod (src/compile/references.js) — set("k", v), put("k", v), and the setX(v) accessor (baseRef[id.slice(3)] = ...) assigned to a computed key without the same check the read paths use. Each computed key is now checked before the write.

3. setValue() #set path walk (src/compile/set.js) — the #set directive walks ref.path doing baseRef[key] = val per segment with no check (for example #set($x.__proto__.foo = ...)). Added a local copy of the reserved-name list (no cross-module import) and check every path segment before traversal and write.

The generic method fall-through is left as-is; the existing top-level guard already covers a literal reserved name there.

Tests

Adds tests/reserved-property-names.test.js (matching the existing test style): the getter re-cased path, get("constructor") / get("__proto__"), the #set path walk, and the put / set write cases all resolve to empty and leave Object.prototype untouched, while normal getters / #set / put still work as before.

mocha tests: 166 passing, 0 failing (13 new assertions, no regressions).