perf: [DSM-144] Misc certification optimizations

[alin-at-dfinity]

• Use precomputed empty leaf hash instead of computing it from scratch every time.
• Pass the key to MapTransformFork::mk_tree by reference, don't clone it (especially since it's only used in a couple of cases).
• Simplify CanisterFork.

[alin-at-dfinity]

This speeds up sequential HashTree computation by about 10%. Unfortunately, the effect on concurrent hashing is a lot smaller (1%-2%), because the bottleneck is memory accesses, not computation.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to a6f274f.

Security Overview

• 🔎 Scanned files: 3 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=5ec8eeed-2bc5-4457-a028-b655396b0b23&codeScanTypes=PrScan&tab=issues

Detected Code Changes

| Change Type | Relevant files

... (code changes summary truncated to fit VCS comment limits.)

[schneiderstefan]

Is this gone in all certification versions?

[alin-at-dfinity]

Yes, only the labels in CANISTER_LABELS and CANISTER_NO_MODULE_LABELS are (and were, before this change) populated.

This was left behind from some old certification version, likely because the match statements below still referenced it, so the compiler did not report it as unused.