feat: add data_load_blf — Vector BLF CAN-log loader (+ shared can_decoder)

[facontidavide]

What

Adds data_load_blf — a Vector BLF (Binary Logging Format) CAN-log
DataSource — and extracts the CAN/DBC decoder into a shared common/ lib so the
MF4 and BLF loaders share one copy.

Stacked on #180 (feat/mf4-plugin). Base branch is feat/mf4-plugin
because the shared decoder is extracted from that (unmerged) work — please
merge #180 first, then rebase this onto main.

Motivated by comparing against the reference tool CANScope: BLF was the
biggest automotive CAN log we lacked, and a BLF importer is ~80% reuse of the
DBC decoder built for MF4.

Approach

1. refactor(can) — move can_decoder out of data_load_mf4 into
   common/can_dbc (target pj_can_dbc), with the dbc_parser_cpp+fast_float
   CPM blocks; dbc stays PRIVATE behind the pImpl. Root CMake adds it (both
   modes), gated to its consumers. Behaviour-neutral: MF4 stays green.
2. data_load_blf — blf_frames (lblf → CanFrame adapter), blf_source
   (orchestrator, per-channel DBC mapping), blf_dialog (a DBC file-picker
   per channel).

Library: PetStr/lblf (MIT, C++20,
zlib-only) vendored via CPM. Technica/vector_blf was rejected (GPL-3.0).

Features

• Classic CAN (CAN_MESSAGE/CAN_MESSAGE2), including zlib-compressed containers
• Per-channel DBC mapping (BLF is multi-bus): one CanDecoder per channel;
  decoded signals → one topic per (channel, message) (CAN/ch{N}/{message})
• Standard/extended (29-bit J1939) disambiguation; unmatched frames + channels
  without a DBC counted and reported
• Interactive dialog: a DBC file picker per channel (1-4, via onFileSelected) +
  a preview of which channels the file contains
• Absolute timestamps: measurement-start SYSTEMTIME (→ epoch ns) + per-object
  relative time (10 µs / 1 ns unit bit)

Verification

• Tests: blf_frames (fixture frames), an end-to-end BLF→DBC decode
  (Speed=100 km/h), and the relocated pj_can_dbc_test — all green
• -Werror clean on gcc 15; ASAN-clean (leak detection) on the fixture;
  self-contained .so (system libs only in DT_NEEDED; both data_source +
  dialog vtables)
• Hermetic fixture: a committed 250-byte python-can BLF (test_data/, +
  gen_blf.py) — no third-party binary

Planning

Designed via a written plan plus an independent Codex plan run in parallel;
the two converged (same library choice, common/ extraction, CAN-FD-out scope,
python-can fixtures), which is why this shipped quickly and cleanly.

Scope / limitations (documented in the README)

• CAN FD out of v1 — lblf has no CAN-FD parse struct and the DBC decoder
  rejects >8-byte payloads; CAN-FD/LIN objects are counted as skipped
• Multiplexed CAN signals not decoded (dbc_parser_cpp limitation)
• Dialog exposes channels 1-4; more via the saved channel_dbcs config

Before merge

• Merge feat: add data_load_mf4 — ASAM MF4/MDF4 file loader (mdflib + DBC CAN decode) #180 first; then this rebases onto main
• Full aggregate build + CI (only single-plugin PJ_BUILD_PLUGIN verified locally)
• pj_proto_app E2E smoke (binary not in the dev env)

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