docs: turn the root README into a project landing page

Reduce the root README to project discovery and navigation: what
irtt-rs is, its relationship to upstream IRTT (including a factual
capability comparison checked against upstream v0.9.2), installation,
a short quick start, and links out to detailed documentation. Detailed
CLI behavior, output/measurement semantics, continuous-mode memory
guidance, and server bind/session/negotiation semantics move into new
manpage-shaped docs under docs/ (irtt-rs, irtt-client, irtt-tui,
irtt-server), rewritten as operational guidance rather than copied
verbatim from the old README sections they replace.

docs/irtt-tui.md

@@ -0,0 +1,83 @@
+# irtt-tui
+
+## NAME
+
+irtt-tui - IRTT-compatible terminal UI client
+
+## SYNOPSIS
+
+`irtt-tui` [*OPTIONS*] *[LABEL=]TARGET*...
+
+## DESCRIPTION
+
+`irtt-tui` is a live dashboard over the same probing engine as
+`irtt-client(1)`: a graph and status view instead of a printed event stream.
+At least one target is required.
+
+Target syntax, multiple targets, and pacing are identical to
+`irtt-client(1)` — see that manual for `[LABEL=]TARGET` syntax and the
+`--pacing staggered|burst` option. The full set of negotiation flags
+(`--interval`, `--length`, `--hmac`, `--clock`, `--tstamp`, `--stats`,
+`--sfill`, `--dscp`, `--ttl`, `--loose`) is shared with `irtt-client` as
+well; consult that manual for their meaning.
+
+## CONTINUOUS DEFAULT
+
+Unlike `irtt-client`, **the TUI defaults to continuous mode**
+(`--duration 0`): it runs until you quit. Pass an explicit `--duration` for
+a finite run:
+
+```sh
+irtt-tui <server> --duration 30s
+```
+
+## CONTROLS
+
+| Key | Action |
+| --- | --- |
+| `q`, `Ctrl-C` | Quit |
+| `p` | Toggle display pause (probing continues; only redraws stop) |
+| `g` | Toggle Graph / Dashboard view |
+| `m` | Cycle graph metric |
+| `r` | Clear visible graph history |
+| `←` / `→` | Pan graph |
+| `PageUp` / `PageDown` | Page-pan graph |
+| `Home` / `End` | Jump to oldest / jump to live |
+| `+` / `=` | Zoom graph in |
+| `-` | Zoom graph out |
+| `0` | Reset graph window and zoom |
+
+## RETAINED HISTORY
+
+The TUI keeps its own bounded presentation state on top of the statistics
+retention described in `irtt-client(1)`:
+
+- Up to 100,000 graph samples per target.
+- Up to 80 recent status/log messages.
+
+`r` clears the currently visible graph history and returns the viewport to
+live; it does not change the 100,000-sample cap itself. These are
+presentation bounds on top of the client's own statistics retention (see
+`irtt-client(1)` MEASUREMENTS AND MEMORY) — a long continuous TUI session's
+total memory is dominated by whichever of the two is larger for your target
+count.
+
+## EXIT BEHAVIOR
+
+Quitting with `q` or `Ctrl-C` is an interrupted, successful exit. Otherwise,
+exit status follows the same peer-close and driver-failure rules as
+`irtt-client(1)`.
+
+## EXAMPLES
+
+```sh
+irtt-tui host.example
+irtt-tui eu=host.example
+irtt-tui eu=host-a.example us=host-b.example --pacing burst
+irtt-tui host.example --duration 30s
+```
+
+## SEE ALSO
+
+`irtt-client(1)` for target syntax, pacing, negotiation flags, and
+finite/continuous memory behavior; `irtt-server(1)`; `irtt-rs(1)`.