fix(tracing): page trace chunks by retained byte offsets

[Xuepoo]

Fixes #99. Task: CTX0054.

Scope

• src/tracing.ts: fetchTraceChunk now pages by actual accumulated encoded byte offsets instead of dividing by a fixed chunk ceiling.
• tests/tracing.test.ts: variable-size pagination tests against a reference concatenated stream (raw and structured), UTF-8 boundary offset tests, normalized fragment tests, multilingual multi-chunk reconstruction, and explicit empty/end/appended-tail continuation tests.

Behavior

• Fetch returns the suffix of the containing retained chunk at an integral UTF-8 byte offset; the caller advances by the encoded page byte length.
• Offsets must be nonnegative safe integers at UTF-8 boundaries; invalid offsets fail explicitly instead of corrupting text.
• At end-of-trace, offset equals byte count and returns an empty page with continuation: false.
• No new wire schema, capability, or protocol version; preview/redaction and retained-byte accounting from CTX0051/CTX0053 are unchanged.

Evidence

• Independent reviewer APPROVE (PX0307/PX0308) at head b5616ca: parents 862b44f (pagination checkpoint) and 7676f87 (origin/main), source blob 06eb011a, test blob 4e4a0ef5.
• Full local gates: bun test 360/360 (2884 expectations, 17 files), scoped 22/22, tsc --noEmit, prettier, oxlint, markdownlint, bun run check:types, bun run build, cargo fmt/check/clippy/test (76/76) with offline env.
• gitleaks detect --source . clean before push.

Complexity

O(retained bytes) time per fetch, O(largest stored chunk) transient space.