fix(etcd): make member join replay idempotent

[weicao]

What this fixes

Closes #3234.

memberJoin no longer blindly adds a member on every lifecycle-action replay.
It now reads authoritative membership before mutation, adds an absent member at
most once, and verifies membership again after the add attempt.

Behavior

• Exact existing member: success without mutation.
• Unstarted member with the expected peer URL: success without mutation.
• Same name with another peer URL: fail closed.
• Same peer URL with another nonempty name: fail closed.
• Absent member: one add attempt followed by an authoritative post-read.
• Query failure, malformed fields output, or unobserved registration: fail
  closed with retry classification.

The fields parser validates decimal member IDs, quoted Name values, nonempty
quoted PeerURL values, and duplicate Name fields. Multiple valid PeerURL fields
remain supported.

Verification

• Genuine TDD reds: focused 31 examples / 4 failures for malformed fields,
  then 34 examples / 2 failures for the quoted-empty PeerURL boundary.
• Focused memberJoin ShellSpec: 34/0.
• Full etcd script ShellSpec: 69/0.
• Alice reviewer mutant harness: pass on Bash 5.3 and BusyBox 1.37, including
  production add_member returning rc=1 with zero add calls for malformed
  input.
• BusyBox 1.37: both byte-exact etcd fixtures classify as
  unstarted-registered; legal multiple-PeerURL input classifies as exact.
• Bash 3.2 and 5.3 parse, ShellCheck error severity, Helm lint, and Helm render:
  pass.

Fixture SHA-256 values remain unchanged:

• etcd 3.5.15: 25406312dafba48e31b88157df628f4c1f0691928f9a3b0e804f4930e24cc9d6
• etcd 3.6.12: c950babd452e3243e74a3d74c5584bd4fa26224077d13f62955e0ae04c245769

The full base diff has two intentional git diff --check findings: the final
blank line in each real-output fixture is an exact captured block delimiter and
is protected by the fixture SHA.