fix: allow transient account closure

[bmuddha]

What changed

After undelegation, ER needs to close the local account so its next use can rematerialize it. Engine previously rejected Transient -> Closed, leaving that account resident. This change allows the transition at the same or a newer slot through the existing delete operation; slot regressions and other unsupported transitions remain rejected.

Closes #161

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 4027680 and 770132e.

📒 Files selected for processing (3)

• solana/account/README.md
• solana/account/src/cow/mod.rs
• solana/account/src/tests/account.rs

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📝 Walkthrough

Walkthrough

The account transition validator now allows a Transient account to transition to Closed at the same or a newer slot. The lifecycle table and matrix test document and check this transition.

Priority: ➖ Normal

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 77013

The change enables the intended transient-account deletion while preserving the existing transition safeguards. No concrete merge-blocking risk is evident.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #161. AccountMode::validate_transition permits Transient to Closed when the destination slot is equal to or newer than the current slot. The lifecycle matrix test cover…
Out of Scope Changes check	✅ Passed	The changed files are within issue #161 scope. The implementation changes the transition rule, the test updates the matching lifecycle case, and the README documents the same behavior. No unrelated ch…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (1 skipped: 1 …
Title check	✅ Passed	The title clearly and concisely identifies the main change: allowing transient account closure.
Description check	✅ Passed	The description directly explains the Transient to Closed transition, the slot condition, the existing delete operation, and the retained rejection rules.

✨ Finishing Touches

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