hardening(dap): bound DapFrameDecoder frames and make accumulation linear

[debugmcpdev]

Fixes #402

DapFrameDecoder accepted any finite positive Content-Length — a peer advertising a huge frame made the buffer grow until OOM (a buggy/compromised adapter can OOM the proxy worker; expose_session mirror clients are loopback + token-gated but defense-in-depth says the decoder itself must cap). Growth was unbounded even before any header: a garbage stream with no \r\n\r\n buffered forever. And push() did Buffer.concat per chunk — O(N²) when a large frame arrives in small chunks.

Change

• maxContentLength option (default 64 MB, matching the repo's maxBuffer precedent; env-overridable via DAP_MAX_FRAME_BYTES). Over-cap advertisements → new error context 'overflow' + buffer discard, same recovery contract as 'header'.
• Header-search bound: >16 KB with no header separator → 'overflow' + discard (headers are a handful of short lines).
• Linear accumulation: header bytes live in a small bounded buffer; body bytes accumulate as a chunk list with a byte count, concatenated once per completed frame. No more per-chunk concat of the whole backlog.
• Consumers: MinimalDapClient treats 'overflow' like 'header' (warn + continue, decoder already discarded); DapMirrorServer closes the mirror connection on 'overflow' as it does on 'header' — a client advertising an over-cap frame is equally untrustworthy.

Tests (property suite extended; new cases watched fail first)

• Over-cap Content-Length → 'overflow' reported, payload discarded, decoder recovers on the next push.
• Exactly-at-cap frame decodes (boundary).
• 20 KB of header-less garbage → 'overflow' + recovery (previously buffered forever).
• 256 KB frame delivered in 1 KB chunks reassembles (the O(N²) shape, now linear).
• The existing chunking-invariance/reassembly fast-check properties and malformed-input recovery cases all still pass; two minimal-dap tests that reached into the decoder's old private rawData now assert the renamed internals.
• Full npm test green, npm run lint clean. DAP_MAX_FRAME_BYTES documented in the setup guide.

🤖 Generated with Claude Code

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