Add native 15 m Landsat panchromatic source

[rappjer1]

Agribound can now prepare native 15 m Landsat B8 panchromatic imagery using source="landsat-pan" through the existing source and configuration interfaces.

Closes #6.

Changes

• Merge Landsat 7/8/9 Collection 2 Tier 1 TOA observations over the requested year or date range, apply sensor-specific QA_PIXEL masks and the existing scene cloud threshold, and export B8 at 15 m.
• Replicate PAN into three channels for compatible RGB-input engines; retain the distinct TOA radiometric treatment and source/sensor provenance.
• Add registry/configuration/QA regression coverage, source/setup documentation and the updated workflow diagram.
• Keep Landsat 7 spectral-response differences and SLC-off gaps explicit. Existing landsat surface reflectance and defaults are unchanged. Pansharpening is outside this source addition.

from agribound import AgriboundConfig

config = AgriboundConfig(
    source="landsat-pan", year=2024, gee_project="YOUR_PROJECT"
)

Two small CI prerequisites are included: a cache-metadata test fixture that preserves its assertions without requiring Windows symlink privileges, and a broken FTW documentation citation replaced by official product metadata. No tests are suppressed or link ignores added.

Validation

• Isolated native scope: Windows Python 3.12 core selection 1,769 passed, 152 skipped; local Ubuntu Python 3.12 full engine-extra selection 1,969 passed, 24 skipped.
• Ruff, formatting, notebook synchronization, strict MkDocs and all 2,665 native-scope link checks passed, with zero link errors/timeouts and the existing exclusions.
• The dependent comparison's authenticated check reproduced the 26 matched Landsat scene identifiers and pinned-checkpoint PAN delineation results.

The offline test marker excludes GPU, Earth Engine, slow and network tests. Optional dependencies account for skips. Hosted submitted-commit/macOS checks must still run; local Ubuntu evidence is from WSL.

The comparison helpers, research examples and offline tutorials are in dependent PR #8: #8. Merge this native-source PR first.