FN-5666: exclude base commit from done-task diff ranges

Prevent done-task diff endpoints from counting base-commit-only lineage as task-owned changes.

- add base-boundary handling in done-task merge SHA resolution so `baseCommitSha` is excluded by default and only consulted for boundary checks
- short-circuit `/diff` and `/file-diffs` responses to empty results when resolved done-task SHA is at or below `baseCommitSha`
- export and use an ancestry helper to detect base-boundary cases, while preserving normal and rebase range behavior
- extend done-task diff route tests with FN-5666 cases for no-op merge SHA, lineage association at base commit, normal post-base commits, and rebase ranges
- add a patch changeset for `@runfusion/fusion`

Files changed:
 .changeset/fn-5666-base-commit-exclusive.md        |   5 +
 .../src/__tests__/routes-diff-done-tasks.test.ts   | 217 ++++++++++++++++++++-
 .../src/routes/register-session-diff-routes.ts     |  57 +++++-
 3 files changed, 273 insertions(+), 6 deletions(-)

Fusion-Task-Id: FN-5666
Fusion-Task-Lineage: 782f9875-856a-4d31-9505-0bcd638c9c29

.changeset/fn-5666-base-commit-exclusive.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix per-task diff view incorrectly including a task's base commit when a done task lands as a no-op or its resolved merge SHA equals `baseCommitSha`.