feat(rnp-src): sync vendored HEAD flavor to librnp main (pin, clone-sync, drop json-c)

[ronaldtse]

Syncs the vendored HEAD flavor (pqc / crypto-refresh) to current librnp main.

What changed upstream

librnp main moved heavily since our last validated sync: json-c was removed in favor of a vendored single-header nlohmann/json (rnpgp/rnp@4f5c4e6e), dump gained decompression-bomb caps, PQC subkey export was fixed (both traditional and PQC subkeys now exported in rnp_key_export_autocrypt), PQC subkey handling is properly #if defined(ENABLE_PQC)-guarded, plus Android/OHOS CI and CMake hardening. The public FFI surface delta vs 0.18.1 is unchanged from what we already track (rnp_op_encrypt_enable_skesk_v6 new, the two sphincsplus functions removed — both already handled in the parity table).

What this PR does

1. Pin the HEAD flavor to 470695b98abe8a427fc47847acb387c089cb156d (current main tip). RNP_HEAD_REF was nominally "pin to a commit for reproducibility" but actually floated main — a fresh build got whatever landed that day, including unvalidated regressions.
2. Sync cached clones to the pin: prepare_librnp_head now fetches + hard-checks-out the pin (with submodule sync) on every build instead of cloning once and reusing a stale tree forever. The Botan-include patcher re-runs afterwards (it is idempotent; --force discards its previous edits).
3. Cache key embeds the pin: Flavor::Head.cache_dir() is now rnp-head-<short-sha> (was the pin-agnostic rnp-flavored), so bumping the pin automatically invalidates stale cached artifacts — the same protection rnp-0.18.1-b1 gives the release flavor.
4. Drop json-c for the HEAD flavor: upstream vendored nlohmann/json, so json-c is neither built nor passed to CMake nor linked for HEAD; Installed.flavor carries the decision and rnp-sys gates static=json-c (and Windows advapi32, which only json-c needed) accordingly. The 0.18.1 release flavor still builds and links json-c exactly as before.

Validation

• rnp-src flavor unit tests: 11 pass (including new pin-is-full-SHA and needs-json-c mapping tests).
• Local --features vendored,pqc,crypto-refresh build: clone synced to 470695b, install/rnp-head-470695b9/lib/librnp.a produced, zero json-c references in the build, link succeeded.
• CI job pqc + crypto-refresh (ubuntu) is the authoritative gate for the flavored build.
• Note: the flavored build segfaults in signing on macOS/ARM64 locally — control experiment (a worktree of main before this PR, same machine) crashes identically, and it reproduces against the 2026-08-28 pin whose ubuntu CI was green, so it pre-dates this sync and is macOS-specific; filed as Flavored (HEAD) vendored build segfaults in signing on macOS/ARM64 — pre-existing, not the pinning sync #100.