compiler: make manifest/import header ordering deterministic for stable lock output

[davidahmann]

Problem

Lock output can drift in non-semantic ways when manifest-style header sections (ImportedFiles / IncludedFiles) are emitted in varying order. This creates noisy diffs and undermines deterministic compile evidence.

Expected behavior

Manifest header sections should be emitted in a stable, canonical order independent of input slice ordering.

Scope (Phase 1)

• Canonicalize ordering of imported/included file lists in workflow header generation.
• Add regression tests that prove deterministic output for mixed-order inputs.

Acceptance criteria

• Header output always emits imports/includes in sorted order.
• Regression test fails on non-deterministic ordering and passes with fix.
• No behavior change outside header rendering.