docs(js-sdk): Opened.decrypt({uuid}) may return DecryptDataResult

[dobby-coder]

Closes #89

Summary

• Documents the behaviour from encryption4all/postguard-js#67 (merged 2026-05-13): Opened.decrypt({ uuid }) now returns DecryptDataResult when the payload was uploaded with Sealed.upload({ data }), and DecryptFileResult otherwise.
• Updates the two-input table in docs/sdk/js-decryption.md to note that { uuid } mirrors the upload mode.
• Rewrites the "Decrypt from Cryptify UUID" section to present DecryptResult as a union, explains the data.bin discriminator briefly, and shows a runtime narrowing example using 'plaintext' in result.
• Adds a one-line note to the docs/repos/postguard-js.md snippet pointing at the SDK page for the data-mode alternative.
• Adds a source link pinned to merge commit 146a7ab70ea8acc6071a4c773a8ae467c1c391a9.

Verification

• npm run docs:build clean (~6s).
• Source link curl -sI returns 200.

Reviewer quickstart

`git fetch origin && git checkout docs/js-decrypt-uuid-data-result && npm install && npm run docs:dev`

[dobby-coder]

Rule compliance check: replaced four em dashes added by this PR with parentheses/periods/colons.

• docs/sdk/js-decryption.md table row: mirrors the upload mode — see below → mirrors the upload mode (see below)
• docs/sdk/js-decryption.md code-block comments: // DecryptDataResult — payload was… → // DecryptDataResult: payload was… (same for the DecryptFileResult branch)
• docs/repos/postguard-js.md: Narrow with 'plaintext' in result — see [Decryption — Decrypt from Cryptify UUID](…) → Narrow with 'plaintext' in result. See [Decrypt from Cryptify UUID](…) for details.

Rule: PostGuard writing style rules — em dashes target zero, hard max 1 per 1,000 words. Pre-existing em dashes elsewhere in these files were not touched (out of scope).

npm run docs:build clean (~6s).

[dobby-coder]

Self-review (cannot --approve own PR — verdict: would-approve).

• Diff matches PR description: union return type documented, two-input table updated, narrowing example added, cross-link on postguard-js.md snippet.
• Source link verified: curl -sI returns 200, and lines 99-121 at 146a7ab70ea8acc6071a4c773a8ae467c1c391a9/src/opened.ts contain the files.length === 1 && files[0] === 'data.bin' unwrap path described in prose.
• Style: no em dashes in the added text (already swapped in the prior compliance pass), no banned vocabulary.
• npm run docs:build clean (~6s) locally; CI build green.