docs: feature spec (v0.2-v0.4)

[dropdevrahul]

Overview

Adds a feature specification document covering four development bundles across milestones v0.2 through v0.4.

Feature Bundles

• A — Response insight: Response headers display, syntax-highlighted JSON/HTML/XML body rendering, status code indicators, response timing, size metrics, and copy-to-clipboard.
• B — Request power: Auth helpers (Bearer, Basic, API Key), multi-part form data support, file upload, cURL import/export, and request history.
• C — Persistence: Local save/load of requests and collections, import/export (Postman/Insomnia compatible), request naming and organisation.
• D — Environments & variables: Named environments, variable interpolation in URLs/headers/body, secret masking, and environment switching.

Milestone Sequencing

Milestone	Bundles
v0.2	A (Response insight) + B (Request power)
v0.3	C (Persistence)
v0.4	D (Environments & variables)

Open Questions

The doc captures several decisions still to be made: storage backend (JSON files vs SQLite), multipart timing behaviour, secrets at rest handling, and navigation UX between saved requests.