docs: add incremental PR plan for test and parser refactor

Author: KristjanESPERANTO

PR_PLAN.md

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+# Incremental PR Plan for Remaining Improvements
+
+This document outlines remaining improvements to be implemented in small, reviewable Pull Requests.
+
+---
+
+## v1.0.0 Candidates
+
+### PR3: parseICS/parseICSAsync API Split
+
+**Branch:** `async`
+
+**Context:** Issue #144 (uncatchable errors in async mode) was fixed in #462 via proper try-catch
+placement. What remains is the cleaner API split itself — see [#476](https://github.com/jens-maus/node-ical/issues/476) for the full motivation.
+
+**Step 1 — non-breaking (do now):**
+- Add `parseICSAsync(str)` as a dedicated async function
+- Keep `parseICS(str, cb)` working but emit a deprecation warning when called with a callback
+- Update `node-ical.js` internal callers to use `parseICSAsync` where appropriate
+- Update TypeScript definitions
+
+**Step 2 — breaking (v1.0.0):**
+- Remove the callback path from `parseICS` entirely
+- `parseICS(str)` becomes sync-only
+- Callers using `ical.parseICS(data, callback)` must migrate to `parseICSAsync`
+
+**Breaking Changes:** Step 1: none. Step 2: yes.
+
+---
+
+### PR4: RFC-Compliant DTEND Handling
+
+**Context:** The parser auto-calculates `end` when `DTEND` is absent, which deviates from RFC 5545.
+
+**Scope:**
+- v1.0.0: Change default behavior to RFC-compliant (`end` is `undefined` when absent)
+- Provide `options.legacyDtend` flag to restore old behavior during migration
+- Add tests for both presence/absence of `DTEND`
+
+---
+
+### PR5: Full ESM Migration with CJS Compatibility
+
+**Scope:**
+- Convert to ESM with dual `exports` field (`import` + `require`)
+- Update TypeScript definitions for both module systems
+- README: ESM-first examples, CJS as legacy
+
+**Breaking Changes:** None for users (dual export). Contributors need ESM knowledge.
+
+---
+
+## Notes
+
+- Keep each PR atomic and independently mergeable
+- Near-term PRs must be fully backward compatible
+- Run full test suite and linter before each PR