[dtt-github]

What

In an example's What actually happened timeline, any value can now be clicked. A Why? panel explains it:

led = 1 from cycle 4: line 20 of blinker.v wrote it: led <= ~led. It ran because !rst_n was false (rst_n = 1), and !en was false (en = 1), and cnt == N - 1 was true (cnt = 3). It used led = 0.

Every value mentioned can be clicked in turn, building a chain back to the cause: through block ports, to the example step that set an input ("set by step 1 of the example"). When the person then asks the AI to fix the design (or the assistant to fix the example), the chain goes with the request. Runs in the browser; no AI, no cost.

How

• Simulator (wafer-sim): elaboration gives each assignment (and port connection, continuous assign, declaration initialiser) an Origin: file, line, scope, text as written, the signals it reads, and the enclosing if/case decisions with the way each went. Op::From(n) marks it; the simulator carries it with each write and records it per change in WaveSignal::by, alongside SimResult::origins. A starting value an assignment confirms without changing is credited to it.
• WASM: by(i) and origins() (JSON).
• Web: lib/why.ts follows a value at a time to its origin (port hops followed; nonblocking assignments read just before the clock edge); Replay exposes the simulation, check times and cycleAt; the timeline rows are clickable and the WhyPanel shows the chain.

Checked

• New simulator test: changes in an up/down counter name the right line, reads and decisions (if/else chain, case arm), through ports to the testbench; starting values are credited.
• Browser, local: the blinker example traced led → cnt → cnt → en (step 1); the UART receiver traced valid → state 3 → state 2 → rx through its case arms.
• cargo fmt --check, clippy -D warnings, cargo test --workspace, web typecheck and build.

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