[config] Execution modes — strict/seeded/adaptive determinism control

[dgenio]

Context

Split from #45. Adds an explicit Mode enum controlling determinism guarantees.

Proposal

class Mode(str, Enum):
    strict = "strict"       # Full determinism (current behavior, default)
    seeded = "seeded"       # Deterministic with explicit seed parameter
    adaptive = "adaptive"   # FUTURE: learns from telemetry (placeholder)

• strict: Current behavior. Identical inputs always produce identical outputs.
• seeded: Deterministic with an explicit seed parameter, allowing reproducible but varied runs.
• adaptive: Future placeholder — no implementation needed now.

Acceptance Criteria

• Mode enum in config.py with strict, seeded, adaptive values
• ProfileConfig.mode field (default: Mode.strict)
• Router respects mode — strict uses current deterministic logic; seeded uses seed for tie-breaking
• adaptive raises NotImplementedError if used (future placeholder)
• to_dict() / from_dict() handles Mode serialization
• Backward-compatible: omitting mode defaults to current deterministic behavior
• Tests for each mode value

File Paths

• src/contextweaver/config.py (edit)
• src/contextweaver/routing/router.py (edit — respect mode)
• tests/test_config.py (edit)

Parent Issue

Part of #45 (ProfileConfig).

[dgenio]

Deferred — speculative future work with no current demand. Will reopen when prerequisites emerge.