fix: make ctxFact gob encoding deterministic

[DerekTBrown]

Summary

• Add deterministic gob serialization for ctxFact
• Sort fact map keys before encoding to avoid Go map iteration order in analysis fact bytes
• Decode the stable entry representation back into the existing map shape
• Add regression tests for byte-stable encoding and round-trip decode

Why

ctxFact is exported as a Go analysis fact. Raw gob encoding of map[string]resInfo can produce different byte streams for the same fact, which is problematic for Bazel/rules_go nogo .facts outputs and content-addressed caches.

Fixes #30

Validation

Red/green evidence:

• Unpatched upstream origin/master / v1.1.6 at 6e20d9954cd54de0d21c93ddc3b98d1fb0493187: standalone regression test failed with gob encoding changed on iteration 0
• Patched branch at 6939d14b5a5f7da390bf88d2c68bf193e94088b2: same standalone regression test passed

Existing validation:

• go test -run 'TestCtxFactGob' -count=1 -v
• go test ./...
• go test -run TestCtxFactGobEncodingIsDeterministic -count=50