refactor(agent): unified transcript block finalization logic

- Replaced commit-based stability checks with a unified `isTranscriptBlockFinalized` tracking mechanism.
- Removed deprecated provisional rendering configuration and flags across tool and renderer interfaces.
- Standardized native scrollback boundary logic to pin at the first unfinalized block using settled row verification.
- Updated and refactored test suites to validate block finalization and settled row boundaries instead of deprecated commit stability methods.

packages/coding-agent/test/transcript-streaming-commit-repro.test.ts

@@ -4,8 +4,11 @@ import type { Component } from "@oh-my-pi/pi-tui";
 
 class MutableLiveBlock implements Component {
 	#lines: string[];
-	constructor(lines: string[]) {
+	#settledRows: number;
+
+	constructor(lines: string[], settledRows: number) {
 		this.#lines = [...lines];
+		this.#settledRows = settledRows;
 	}
 	render(width: number): string[] {
 		return this.#lines.map(line => line.slice(0, width));
@@ -16,22 +19,26 @@ class MutableLiveBlock implements Component {
 	isTranscriptBlockFinalized(): boolean {
 		return false;
 	}
+	getTranscriptBlockSettledRows(): number {
+		return this.#settledRows;
+	}
 }
 
 describe("transcript streaming commit (assistant text)", () => {
-	it("treats in-place growth of the trailing line as append-only", () => {
+	it("commits only the declared settled head while the trailing line grows", () => {
 		const chat = new TranscriptContainer();
 		// Models a streaming assistant reply: stable head rows plus a current
-		// line that grows token-by-token without adding a new row.
-		const block = new MutableLiveBlock(["para one", "para two", "the quick brown"]);
+		// line that grows token-by-token without adding a new row. The head is
+		// committable only because the block explicitly declares those rows settled.
+		const block = new MutableLiveBlock(["para one", "para two", "the quick brown"], 2);
 		chat.addChild(block);
 
 		chat.render(80);
+		expect(chat.getNativeScrollbackLiveRegionStart()).toBe(2);
 
 		block.setLines(["para one", "para two", "the quick brown fox"]);
 		chat.render(80);
-		// The head rows never changed; only the trailing line grew. Its scrolled-
-		// off head must be committable to native scrollback (tmux pane history).
-		expect(chat.getNativeScrollbackCommitSafeEnd()).toBe(3);
+
+		expect(chat.getNativeScrollbackLiveRegionStart()).toBe(2);
 	});
 });