EPICS PVA multi-root with N PVI roots

[gilesknap]

Parent

#351

What to build

EPICS PVA hosts N controllers in one process, each exposing its own :PVI root for browsing with existing PVI tooling. There is no super-parent PVI record — the PRD explicitly omits this and notes it can be revisited if discovery tooling demands it.

Scope:

• EPICS PVA transport adopts the multi-controller connect() properly: N independent PVI roots, one per configured controller.
• Reuses the D2 PV-prefix derivation utility (already introduced in Multi-controller foundation end-to-end via REST transport #353 and adopted by CA in EPICS CA multi-root softioc with id-based PV prefix #354).
• Per-transport PVA id validator runs at connect() time: rejects illegal characters and the 60-char overflow case.
• Extend tests/test_multi_controller.py with the EPICS-PVA scenario: two controllers each get their own PVI root with no clash.
• PVA transport test gains a per-transport id-validation case demonstrating the fail-fast behaviour.

User stories from #351 covered: 12, 16, 17.

Acceptance criteria

• One process exposes N independent :PVI roots, one per controller, no super-parent
• PVA id validator rejects illegal characters and 60-char overflow with clear startup errors
• tests/test_multi_controller.py covers the two-controller PVI scenario over PVA
• PVA transport test covers id-validation fail-fast
• Full test suite stays green

Blocked by

• EPICS CA multi-root softioc with id-based PV prefix #354