fix(rne): don't crash when a joint's kinematic parent is a base-mounted static link

[petercorke]

Problem

Follow-up to #693. While cleaning up the dev conda environment (unrelated
to this fix) and smoke-testing `rne()` against a real model rather than
just synthetic test topologies, hit an `IndexError` on the default
`Panda()`:

```
IndexError: list index out of range
group_idx = [
i for i, group in enumerate(link_groups) if parent_idx in group
][0]
```

Root cause: `panda_link0` is a static base link with no joint ancestor at
all (`isjoint=False`, `parent=None`) -- #693's grouping correctly drops
it from `link_groups` entirely, since it contributes no joint torque
(rigidly fixed to the immovable base). But the first joint's own
`.parent` is that static link, and resolving its upstream kinematic
group via a raw list-membership search over `link_groups` breaks once
that link is (correctly) a member of no group.

Fix

Reuse `group_of_link_idx` (already built once by #693's grouping pass)
to resolve a joint's nearest joint ancestor directly. "No entry" (no
joint ancestor upstream at all) is now treated the same as `.parent`
being `None` outright, rather than crashing.

Verification

• Red/green: reverted just the `Robot.py` fix (keeping the new test) and
  confirmed it fails with the exact same `IndexError`; restored the fix
  and confirmed it passes.
• New regression test (`test_invdyn_base_mounted_static_link`) isolating
  this exact topology.
• Full suite: 745 passed, 0 failed, 82 skipped
• Smoke-tested against the real `Panda()` and `Frankie()` models (both
  previously crashed on this path, both now compute correctly)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (7aa2d31) to head (09c5b15).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/roboticstoolbox/robot/Robot.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #696   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        143     143           
  Lines      14091   14089    -2     
=====================================
+ Misses     14091   14089    -2     

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