feat(cli): --lower-dtr / --lower-rts (picocom-compat startup line control)

[TrekMax]

Motivation

picocom -b 921600 /dev/ttyACM0 --lower-dtr --lower-rts is the canonical
"open the port without resetting the MCU" recipe for embedded developers
working with Arduino-style boards (where DTR/RTS are wired to the
reset / boot pins). Today rtcom has no equivalent, forcing users to
either:

1. Open the port (which asserts both lines via the OS default), then
   manually press ^A t and ^A g to deassert. Two extra
   keystrokes plus a brief high-level pulse on each line — enough to
   reset some boards.
2. Live with the reset.

Neither matches what picocom --lower-dtr --lower-rts does atomically
right after open().

Proposal

Add four boolean CLI flags mirroring picocom 1:1:

• `--lower-dtr` — deassert DTR immediately after opening the device.
• `--lower-rts` — deassert RTS immediately after opening the device.
• `--raise-dtr` — assert DTR immediately after opening the device.
• `--raise-rts` — assert RTS immediately after opening the device.

`--lower-X` and `--raise-X` for the same line are mutually exclusive
(clap can enforce via `conflicts_with`). Default behaviour is unchanged
from today (whatever the OS / driver gives us at `open()`, which is
typically both lines asserted).

Implementation sketch

`crates/rtcom-cli/src/args.rs`:

```rust
/// Deassert DTR immediately after opening the device.
#[arg(long, conflicts_with = "raise_dtr")]
pub lower_dtr: bool,

/// Assert DTR immediately after opening the device.
#[arg(long, conflicts_with = "lower_dtr")]
pub raise_dtr: bool,

// ... rts equivalents ...
```

`crates/rtcom-cli/src/main.rs::async_main` — between
`SerialPortDevice::open` and `Session::new`:

```rust
if cli.lower_dtr { device.set_dtr(false)?; }
else if cli.raise_dtr { device.set_dtr(true)?; }
if cli.lower_rts { device.set_rts(false)?; }
else if cli.raise_rts { device.set_rts(true)?; }
```

`crates/rtcom-core/src/session.rs` — accept the initial DTR/RTS state
via builders so the cached `dtr_asserted` / `rts_asserted` matches
reality and the first `^A t` toggle does the right thing:

```rust
session = session.with_initial_dtr(false).with_initial_rts(false);
```

Or simpler: have main set the lines directly and call
`session.with_initial_lines(dtr, rts)` (one builder, two args).

Acceptance

• `rtcom /dev/ttyACM0 -b 921600 --lower-dtr --lower-rts --imap crlf`
  runs cleanly.
• After startup, `^A t` flips DTR back to asserted (banner says
  `DTR: asserted`); next `^A t` deasserts again.
• `--lower-dtr --raise-dtr` returns a clap error rather than
  doing the second one wins.
• Unit tests in `args::tests` cover defaults (all four false),
  single-flag parses, mutual exclusion errors.
• Integration test in `session_loopback.rs` sets initial lines
  via builder and asserts `session.dtr_asserted` / `rts_asserted`
  reflect the value.
• README + man page list the four flags with picocom-equivalence
  callouts.
• CHANGELOG `[Unreleased]` entry.

Out of scope (separate issue if anyone asks)

• `--dtr=low|high|toggle` value-style flag. More flexible but the
  boolean pair already covers the picocom muscle-memory case; defer
  until someone needs the runtime-toggle-at-startup variant.
• Honouring `--no-reset` semantics. `--no-reset` currently means
  "don't toggle DTR at startup"; with this issue landed it can be
  documented as equivalent to `--lower-dtr` for Arduino-reset wiring.

References

• picocom(1) man page, `--lower-dtr` / `--lower-rts` / `--raise-*`
• Real-hardware session that surfaced the gap: see the smoke-test
  log on local dev, late v0.1 polish.

Tracker: planned for v0.2.

[TrekMax]

Implementation summary (auto-closed by the "Closes #1" reference in the CHANGELOG):

Commits

• 5acfa7e feat(cli): --lower-dtr / --raise-dtr / --lower-rts / --raise-rts (args + 8 tests)
• b8e0ed6 test(core): Session needs with_initial_dtr / with_initial_rts (red)
• 1ce33db feat(core): Session::with_initial_dtr / with_initial_rts (green)
• 159e6d8 feat(cli): wire --lower/raise-dtr/rts into device + Session
• dc84a7e docs: --lower/raise-dtr/rts user-facing surface

Acceptance

• ✅ `rtcom /dev/ttyACM0 -b 921600 --lower-dtr --lower-rts --imap crlf` runs cleanly
• ✅ After `--lower-dtr`, the first `^A t` raises DTR (test: initial_dtr_lowered_first_toggle_yields_asserted); same for RTS
• ✅ `--lower-dtr --raise-dtr` returns a clap conflicts_with error
• ✅ args::tests: defaults (4 false), single-flag parses, mutual exclusion, DTR/RTS-axis crossing
• ✅ Session integration tests verify cached state via observable toggle behaviour
• ✅ README + man page updated with picocom-equivalence + EXAMPLES entry
• ✅ CHANGELOG [Unreleased] entry

Ships in v0.1.2.