fix(rune): address conductor migration nitpicks

[martin-henz]

Re-opens #835, which was automatically closed (not merged) when its base branch conductor-migration was deleted after being merged into master via #680. This branch cherry-picks the original commit from @11suixing11 unchanged, onto current master.

Summary

• validate the rune channel before calling the base plugin constructor
• make rune initialisation idempotent so primitive rune exports are not duplicated
• load the host tab by the shared Rune tab name instead of by the first tab entry
• share rune deserialisation through the bundle protocol and cover serialize/deserialize round trips

Addresses #827

Motivation

This PR follows #827, which came from checking which CSG conductor-migration review findings also apply to Rune. It is scoped to the four Rune items from that issue and intentionally does not address the separate duplicate-delivery or js-slang error-shape issues tracked in #823 and #825.

Concrete Cases

• Constructor guard: the old constructor passed [undefined] into BaseModulePlugin before throwing Rune's own missing-channel error. Moving the guard before super(...) keeps invalid channel state out of the base plugin.
• Initialisation idempotency: calling await plugin.initialise() twice re-ran super.initialise() and re-pushed method and primitive-rune exports. The new guard wraps the full method so repeated initialisation leaves the export list stable.
• Tab lookup: __loadRuneTab() previously used tabs[0]. Looking up RUNE_TAB_NAME keeps the tab contract explicit if the host ever provides more than one tab.
• Protocol round trip: the tab previously had its own local deserializeRune. Sharing it from protocol.ts lets the bundle test the same serialization/deserialization path used by the tab.

Testing (from original PR, re-verify on CI)

• yarn workspace @sourceacademy/bundle-rune test (47 passed)
• yarn workspace @sourceacademy/bundle-rune tsc
• yarn workspace @sourceacademy/bundle-rune lint
• yarn workspace @sourceacademy/tab-Rune test (3 passed in Chromium)
• yarn workspace @sourceacademy/tab-Rune tsc

Original author: @11suixing11

[gemini-code-assist]

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