Fix rawspeed-cli ARW/CR2/PEF decoding (missing metadata pass)

[Abas-Tim]

Summary

Fixes the v1.0.1 regression: ARW/CR2/PEF decoding failed with
rawspeed: unsupported CFA pattern (exit 1).

Root cause: main.cpp never called decoder->decodeMetaData();
those decoders set the CFA in their metadata pass
(ArwDecoder.cpp:501, same for Cr2/Pef), so render() saw a 0x0 CFA
and failed. The v1.0.0 CLI only worked because it unconditionally
forced an RGGB CFA in render() — a load-bearing call that the
clang-tidy cleanup removed as apparent dead code. Full details in
docs/rawspeed-cli-v1.0.1-regression-report.md.

Changes:

• call decoder->decodeMetaData(&meta) after decodeRaw();
  cameras.xml is resolved like rawspeed-identify does
  (RS_CAMERAS_XML_PATH → <bindir>/../share/darktable/rawspeed/cameras.xml
  → RAWSPEED_SOURCE_DIR/data/cameras.xml), falling back to an empty
  CameraMetaData{} (the decoders set RGGB before consulting the DB,
  and unknown cameras only warn with failOnUnknown = false)
• a failing metadata pass is logged and skipped instead of aborting
  the decode
• render() hardening: CFA smaller than 2x2 now defaults to RGGB
  instead of failing; larger patterns (e.g. X-Trans) still fail as
  unsupported; the 4-Bayer pattern mapping is unchanged
• publish smoke test now decodes a real image: a synthetic 8x8
  uncompressed RGGB CFA DNG generated in the runner, verifying the
  PPM header and size (the usage-only check cannot catch this class
  of regression)
• regression report added under docs/

Verification

• synthetic 8x8 CFA DNG: exit 0, P6 8 8 65535, 397 bytes (8-bit: 203)
• usage path: exit 2 with usage text (unchanged)
• clang-format 18.1.8 idempotent; clang -Weverything -Werror clean;
  clang-tidy (CI check set) clean; MSVC Release build green; exact CI
  smoke-test script executed locally against the fresh build

Notes

• Prepared with AI assistance (opencode), per AGENTS.md disclosure rules
• Fixes the release tracked in rawspeed-cli-v1.0.1; v1.0.2 will be
  tagged after merge