chore(cbind): nim-ffi migration [4/9] — streams & custom protocols (+ echo example)

[gmelodie]

nim-ffi cbind migration — PR train

1. chore(cbind): nim-ffi migration [1/9] — deps + pinning scaffold #2772 — deps + pinning
2. chore(cbind): nim-ffi migration [2/9] — new-library scaffold + CI #2773 — scaffold + CI
3. chore(cbind): nim-ffi migration [3/9] — connectivity & identity #2774 — connectivity
4. chore(cbind): nim-ffi migration [4/9] — streams & custom protocols (+ echo example) #2775 — streams (+ echo) ← this PR
5. chore(cbind): nim-ffi migration [5/9] — pubsub / gossipsub (+ gossipsub example) #2776 — pubsub (+ gossipsub)
6. chore(cbind): nim-ffi migration [6/9] — kademlia + CID + crypto #2777 — kad + CID + crypto
7. chore(cbind): nim-ffi migration [7/9] — service discovery + extended peer records #2778 — service discovery
8. chore(cbind): nim-ffi migration [8/9] — relay, peerstore, metrics (API parity) #2779 — relay / peerstore / metrics
9. chore(cbind): nim-ffi migration [9/9] — flip to libp2p.nim, delete legacy cbind #2780 — flip + delete legacy

---

Part 4 of a 9-PR train that replaces the hand-rolled cbind/ C bindings with the generated nim-ffi codegen, landing it domain by domain so each PR stays reviewable and CI-green (build the FFI lib + generate bindings + build/run examples).

Targets chore/cbind/ffi/03-connectivity (stacked).

This PR adds

• Stream ops over streamId handles: readExactly, readLp, write, writeLp, close, closeWithEof, release.
• libp2pMountProtocol + the onIncomingStream {.ffiEvent.}, plus the full StreamRegistry release-waiter logic that keeps custom-protocol handlers alive across the FFI boundary.
• validateReadLength hardening — rejects negative lengths, the MaxReadBytes ceiling, and values that would narrow-overflow a 32-bit int (matters for linux-i386 CI).
• The echo C example plus the examples build harness: common.h, CMakeLists.txt, README.md, .clang-format, updated Makefile, and the examples nimble task.
• Retires the clang-format linter step (the C files it guarded are rewritten here).

Review artifact: Header diff shows the stream/protocol entry points and the incoming-stream event.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (chore/cbind/ffi/03-connectivity@2704369). Learn more about missing BASE report.

Additional details and impacted files

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