EPICS CA multi-root softioc with id-based PV prefix

[gilesknap]

Parent

#351

What to build

One softioc hosts N controllers, each with its id as the verbatim PV prefix. EPICS path-to-PV mapping treats path[0] (the id) verbatim; subsequent path segments continue through snake-to-Pascal conversion. The previous EpicsIOCOptions dataclass and its pv_prefix field are removed entirely — the id is the prefix.

Scope:

• EPICS CA transport adopts the multi-controller connect() signature properly: hosts all configured controllers in one softioc.
• Adopts the D2 PV-prefix derivation utility introduced in Multi-controller foundation end-to-end via REST transport #353 (path[0] verbatim, path[1:] snake-to-Pascal). The previous separate prefix parameter to the PV-prefix utility is removed.
• EpicsIOCOptions and its pv_prefix field deleted; demo and tests updated accordingly.
• Per-transport CA id validator runs at connect() time: rejects illegal characters AND emits a clear error when the id plus longest path exceeds the EPICS 60-character PV name limit.
• tests/conftest.py pv_prefix fixture migrated to id-based naming.
• Extend tests/test_multi_controller.py with the EPICS-CA scenario: two controllers wire into one IOC with distinct PV prefixes and no clash.
• CA transport test gains a per-transport id-validation case demonstrating the fail-fast behaviour at connect() time.

User stories from #351 covered: 16, 17, 18, 32, 34.

Acceptance criteria

• One softioc serves all configured controllers; each controller's PVs appear under its verbatim id
• D2 PV-prefix utility adopted; standalone prefix parameter removed
• EpicsIOCOptions.pv_prefix removed; no callers remain
• CA id validator rejects illegal characters and 60-char overflow with clear startup errors
• conftest.pv_prefix fixture migrated to id-based naming
• tests/test_multi_controller.py covers the two-IOC-no-clash scenario over CA
• CA transport test covers id-validation fail-fast
• Full test suite stays green

Blocked by

• Multi-controller foundation end-to-end via REST transport #353