[TASK] FastWalker protocol + default exclusion constants

[kirich1409]

Context

Public protocol in DevsweepCore. defaultExcludedBasenames (Set) and defaultExcludedPathPrefixes ([URL]) as composable public constants.

Acceptance criteria

• Protocol signature per research §FastWalker protocol.
• Default sets exposed and composable (subtract/union).

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

None.

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved