brontide: use tiered buffer pool for encrypted message bodies

[kpshukla]

Summary

Fixes #10246.

bodyBufferPool (added in #10183) always allocates a max-size (65,551
byte) buffer for every outgoing message body, to avoid reallocation.
In practice, most LN wire messages (gossip updates, HTLC messages,
etc.) are well under that size, so the max-size allocation is wasted
memory for the common case.

This PR replaces the single pool with a tieredBufferPool: five
sync.Pools bucketed at 256B / 1KB / 4KB / 16KB / max
(maxMessageSize). WriteMessage already knows the plaintext length
up front, so it now requests a buffer sized to len(p) + macSize
instead of always taking the largest tier. Buffers are returned to the
pool matching their exact capacity; a buffer whose capacity doesn't
match any tier (shouldn't happen in practice) is discarded rather than
risk polluting a tier with an oddly-sized buffer.

headerBufferPool is untouched — encrypted headers are a fixed 18
bytes (encHeaderSize), so there's no benefit to tiering it.

Test plan

• go build ./... — clean, no downstream breakage in packages that
  depend on brontide.
• go test ./brontide/... — full existing suite passes unchanged,
  including:
  • TestConnectionCorrectness, TestConcurrentHandshakes
  • TestMaxPayloadLength (boundary at math.MaxUint16)
  • TestWriteMessageChunking, TestFlush (partial-write / resume paths)
  • TestBolt0008TestVectors (BOLT#8 spec vectors)
  • All Fuzz* seed corpus cases (FuzzRandomActOne/Two/Three,
    FuzzStatic*, FuzzRandom*EncDec, etc.)
• Added TestTieredBufferPool covering tier selection at
  boundaries (exact match on a tier, rounds up to next tier, falls
  back to largest tier when the request exceeds every tier) and the
  discard-on-capacity-mismatch path.
• gofmt -l and go vet ./brontide/... — clean.

[github-actions]

🔴 PR Severity: CRITICAL

file classification | 3 files | 164 lines changed

🔴 Critical (1 file)

• brontide/noise.go - Modifies the Noise protocol / encrypted peer connection implementation (brontide is listed as a CRITICAL package: encrypted peer connections, Noise protocol)

🟢 Low (2 files)

• brontide/noise_test.go - Test-only change, excluded from severity-driving classification
• docs/release-notes/release-notes-0.22.0.md - Release notes documentation update

Analysis

The only substantive (non-test, non-doc) file touched is brontide/noise.go, which implements lnd's Noise_XK-based transport encryption for peer connections. Per the severity rules, brontide/* is classified as CRITICAL since it directly affects the security of encrypted peer-to-peer communication. No severity bump was needed: excluding the test file and release notes, this PR touches 1 file and ~86 lines (75 additions / 11 deletions) in noise.go, well under the >20 file / >500 line bump thresholds, and it doesn't span multiple distinct critical packages. The accompanying test additions in noise_test.go are a good sign but don't reduce the need for expert review of the underlying Noise protocol logic change.

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