Set impl_1 strategy before sourcing pre-synth Tcl

[merkelmarrow]

In slash_project_build.tcl, the default implementation strategy was applied to impl_1 after the user's pre-synth Tcl scripts were sourced. Because set_property strategy ... resets every STEPS.* option on the run to the strategy's defaults, any per-step directive a pre-synth script set was silently removed before launch_runs impl_1.

This moves the strategy assignment ahead of the pre-synth sourcing loop so the strategy is the baseline and pre-synth scripts can override individual step options on top of it.

Testing pending.

[merkelmarrow]

Testing

Ran a full slashkit link -p hw build of a single-master RTL kernel with the fix, passing a pre-synth Tcl that overrides the phys_opt and route directives on impl_1:

set_property STEPS.PHYS_OPT_DESIGN.ARGS.DIRECTIVE Explore [get_runs impl_1]
set_property STEPS.ROUTE_DESIGN.ARGS.DIRECTIVE  Explore [get_runs impl_1]

The build log shows the strategy is now applied before the pre-synth Tcl runs, and the override sits on top of the defaults.

PRESYNTH: strategy            = Congestion_SSI_SpreadLogic_high
PRESYNTH: phys_opt directive before = AggressiveExplore   (strategy default)
PRESYNTH: route   directive before = AlternateCLBRouting  (strategy default)
PRESYNTH: phys_opt directive after  = Explore             (override applied)
PRESYNTH: route   directive after  = Explore             (override applied)

The real impl_1 then executed with the overridden directives and the build completed and produced a vbin. In comparison, an earlier build without the fix ran phys_opt_design -directive AggressiveExplore and route_design -directive AlternateCLBRouting, the strategy defaults, with the override discarded.