feat: add redemption windows for holder-initiated share buybacks

[skaichima]

Title: feat: Add redemption windows allowing holders to redeem shares during open windows

Description:

Introduces a complete redemption lifecycle for revenue-share holders:

• set_redemption_window — issuer configures an open window (start_timestamp, end_timestamp) per offering. Unset windows are treated as always-open.
• request_redemption — holder requests to redeem shares_bps of their share. Validates share ownership, blacklist status, and window openness. Stores a pending RedemptionRequest. Emits red_req event.
• fulfill_redemption — issuer transfers amount of the offering's locked payment token to the holder, reducing their share by the requested bps (capped at current share). Emits red_fil event.
• get_redemption_window — read-only query returning the configured window or None.
• require_redemption_window_open — internal guard for both request and fulfill paths.
• 24 tests covering: window management, in-window and out-of-window request handling, always-open default, blacklist rejection, duplicate/zero/oversized share rejection, fulfillment edge cases (zero amount, cap at current share, re-request after partial fulfillment), non-issuer rejection, and event emission.

All CI gates pass (cargo fmt, clippy, build --release, test -- --test-threads=1 — 106/106).

closes #457

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