refactor(catchment): decide what extract_discharge(only_outlet=) should do

[MAfarrag]

Context

Found during a review of extract_discharge. The parameter is public, documented as doing something, and has
never done it. Present on main; documented as inert rather than changed, because fixing it alters behaviour and
deserves its own decision.

Problem / Current Behaviour

Catchment.extract_discharge dispatches like this:

if not frame_work_1:
    ...                      # per-gauge extraction
elif frame_work_1 or only_outlet:
    ...                      # basin-wide sum

The elif is reached only when frame_work_1 is already True, so only_outlet can never change which branch
runs. extract_discharge(only_outlet=True) behaves exactly like extract_discharge().

The docstring says "True to extract discharge only at the outlet cell", so a caller has no way to discover that
the flag does nothing short of reading the dispatch.

Affected locations

File	Symbol	Notes
src/hapi/catchment.py	Catchment.extract_discharge	the elif frame_work_1 or only_outlet dispatch
src/hapi/calibration.py	Calibration.extract_discharge	override; accepts and ignores the same flag

Steps to Reproduce

model.extract_discharge(only_outlet=True)
# identical result to model.extract_discharge() -- the flag selects nothing

Proposed Solution

Decide between three, none of which is obviously right:

1. Make it work — give only_outlet its own branch, independent of frame_work_1. Changes behaviour for
   anyone passing it today, though since it currently does nothing, nobody can be relying on it working.
2. Remove it — drop the parameter from both signatures. Cleanest, but breaks any call site passing it by
   keyword.
3. Keep documenting it as inert — the current state, chosen to avoid a behaviour change inside an unrelated
   refactor.

Whichever is chosen, the Calibration override should follow.

Out of Scope

• The MAXBAS guard in the same method, which is separate and working.

Effort Estimate

Size: XS
Rationale: a few lines either way; the cost is the decision, not the change.

Definition of Done

• A decision is recorded on the issue
• only_outlet either selects the outlet-cell path, or is gone from both signatures
• The docstrings match the behaviour
• Tests cover whichever behaviour is chosen
• All existing tests continue to pass