[DO NOT MERGE] Establish core-v2-maintenance branch for QA testing of rust-core v2

[paul-brackin]

⚠️ DO NOT MERGE. This PR exists for review and discussion only. The
core-v2-maintenance branch is intended to be a long-lived parallel
maintenance line, not a feature branch. Merging it into main would
regress the v3 cutover. Once review feedback (if any) is incorporated, this
PR should be closed without merging.

Why this branch exists

Our QA team uses PAC as the test harness for the Rust client core. Rust-core
v2 is the released version in the field, but PAC's main was migrated to
v3 (afcb863 "Core v3 cutover (#63)") in late April. That cutover added
hard dependencies on v3-only APIs (Txn, ReadModeAP/SC, use_compression,
operations::path), so QA can no longer point PAC at the released v2 core
without it failing to compile.

Symptom QA hit when trying:

error: failed to select a version for `rustls`.
... required by package `aerospike-core v2.1.0 (https://github.com/aerospike/aerospike-client-rust?branch=v2#86224f4)`
... versions that meet the requirements `^0.23.40` are: 0.23.40
... previously selected package `rustls v0.23.37`

That specific error is a stale-Cargo.lock artifact (v2 bumped rustls in
commit acb3cae). The deeper problem is API divergence: even with the lock
refreshed, src/policies.rs, src/lib.rs, etc. on main reference v3-only
symbols.

What this PR does

Creates core-v2-maintenance, branched from fec5d1a (the commit
immediately before the v3 cutover, version 0.3.0-alpha.14, already
pointing at aerospike-core branch = "v2"). Then forward-ports the
non-feature work that landed on main after the cutover.

Cherry-picked from main (post-cutover)

Original SHA	Subject	Notes
753293b	Release tooling (#65)	clean
93e1b06	ci: tighten build-test.yml triggers (#66)	clean
257396e	Harden release.yml: deprecation notice + strict branch gate (#67)	clean
59f0f5d	[StepSecurity] Apply security best practices	clean
b5a95ff	ci: publish-rtd skeleton (Gate 4 RTD)	clean
3ba612e	Rename PyPI dist to aerospike-async; bump to 0.3.0-alpha.17	trivial Cargo.{toml,lock} version conflict — accepted incoming
7605ba7	release: fix MATURIN_REPOSITORY_URL routing; bump to 0.3.0-alpha.18	clean
d202855	Add PyPI project metadata; bump to 0.4.0-alpha.1	clean

Intentionally NOT cherry-picked

• b8c192a CLIENT-4701 "Unified flag input handling..." — too tangled
  with v3 cutover for a clean port. The diff registers v3-only PyO3 classes
  (TxnState, Txn, ReadModeAP/SC, SelectFlags, ModifyFlags),
  modifies the v3-only path-expression test files, and adds v3 MRT result
  codes. Backportable in principle by manually extracting just the generic
  flag-input plumbing (List/Map/Exp flags accepting either int or enum),
  but that's a 30–60 minute focused port, not a cherry-pick. Tracked as
  follow-up if QA reports needing it.

New maintenance-branch commits

• caf07ae — Branding + dist split:
  • README banner clearly labels this as the v2 maintenance branch and lists
    which features (MRTs, path expressions, compression, SC read modes) are
    absent.
  • pyproject.toml: dist renamed aerospike_async → aerospike-async-core-v2
    so it can coexist with the v3 aerospike-async distribution. Python
    import path stays aerospike_async so QA tests don't need to change —
    only the install name differs.
• 1a99448 — Refresh Cargo.lock against rust-core v2 tip (86224f4e).
  This is the fix for QA's rustls error.

Validation

$ cargo check                                   # default features
   Finished `dev` profile [unoptimized + debuginfo] target(s) in 20.87s
$ cargo build --no-default-features --features tls --lib
   Finished `dev` profile [unoptimized + debuginfo] target(s) in 2m 46s

Both green. Resolved against aerospike-core v2.1.0 (sha 86224f4e) and
rustls v0.23.40 — exactly the combo QA was trying to build.

Decisions still open (defer to maintainer)

1. Version scheme. Currently 0.4.0-alpha.1 (inherited from d202855).
   Likely wants its own line — e.g. 0.3.5a1 — before the first publish.
2. Pinning policy. aerospike-core is currently branch = "v2" (floats
   with the v2 tip). If you want deterministic builds across QA machines,
   change to rev = "86224f4" (or any future v2 tag).
3. CI on this branch. The cherry-picked workflows still target the same
   triggers as main. May want to scope them to core-v2-maintenance or
   give them their own job names to keep PR-CI clean.
4. b8c192a CLIENT-4701 trimmed-down backport. Worth doing only if QA
   says they need the unified flag-input handling on v2 testing.
5. Lifecycle. Stated to be indefinite. Flagging only because most
   teams sunset maintenance branches when the supported core is EOL'd.

How QA uses this

# install for testing rust-core v2 (released version)
pip install aerospike-async-core-v2

# OR, cloning + building locally:
git clone -b core-v2-maintenance git@github.com:aerospike/aerospike-client-python-async.git
cd aerospike-client-python-async
maturin develop  # or pip install .

Their import path stays the same: import aerospike_async.